The supplication (dua) is one of the best worship through which one may attain self-perfection and God's Nearness. Because, of this reason God-Almighty has invited his servants to offer supplications. God-Almighty says in Holy Quran:
"And your Lord hath said: Pray unto Me and I will hear your prayer. Lo! Than who scorn My service, they will enter Hell disgraced."
-the Holy Quran (40:60)
("Oh mankind!) Call upon your Lord humbly and in secret. Lo! He loveth not aggressors."
-the Holy Quran (7:55)
"And when My Servants question thee concerning Me, then surely I am nigh. I answer the prayer of the suppliant when he crieth unto Me."
-the Holy Quran (2:186)
The Holy Prophet (S) has said:
"The supplication (Dua) is the soul of the worship."
-Sahih Tirmidhi, vol. 2, p-266.
Imam al-Sadiq (A) has said:
"Supplication is worship, because God-Almighty says: ' You must continue to seek God-Almighty and should never say: It is all done."
-al-Kafi, vol. 2. p-407.
"You should never quit supplication in all circumstances, because you will never find any other substitute like supplication in attaining God's-Nearness. Even, for insignificant  and minor affairs one must supplicate, and because of their being insignificant supplication should not be abandoned, because, after all the Master of petty affairs happens to be the same Master of large affairs."
-al-Kafi, vol. 2, p-467.
God's Servant must supplicate because his entire existence needs God-Almighty. Because, a human being in essence is absolutely poor, needy, and dependent; and in case he becomes deprived of God's blessings even for an instant, he will be destroyed as though he was never existed. Whatever, reaches to a servant is bestowed from God-Almighty, therefore, a servants should admit this primordial dependency by his tongue and should confirm his poverty, servitude, and needs, through his practical actions, which is the real meaning of worship.
At the time of supplication a human being remembers God-Almighty, establish humming communications with him, and like a humble servant with tearful eyes presents his needs before the God-Almighty the Owner of Absolute Riches. By abandoning his hopes from the world of poverty and wants, he establishes his link with the Most Supreme Source of all Blessings and Perfection.
Flying upward from the world of poverty, he succeeds in witnessing the Beauty of God's Essence, through his esoteric eyes. The state of supplication and humming private communications with God-Almighty is one of the most pleasurable and beautiful condition of a servant which will never be exchanged for any price by God's saints.
Refer to al-Sahifah al-Sajjadiyyah  and other supplication books and study the details of humming communications of Infallible Imams (A) of the Prophet's Holy Progeny. Establishing communications with God-Almighty and hoping that He will accept the supplications, brings tranquility, and assurance for the suppliant's heart. If a human being, while facing lives hardships for solution of his problems and difficulties, does not seek refuge in God-Almighty, then how could he show perseverance against them and have assurance for continuation of his life?
Supplication is a believer's arsenal through which he struggles against disappointments and despairs, and seeks help from a hidden super natural power for solution of his difficulties and problems. The Divine-Prophets and Infallible Imams (A) always utilized this arsenal and have recommended it strongly for believers. Imam al-Ridha (A) said to his companions:
“Use the arsenal of Prophets.” What is the arsenal of prophets? He was asked. “Supplication”. Replied the Imam."
-al-Kafi, vol. 2, p-468.
Imam al-Baqir (A) said:
"God-Almighty, among .the believers loves the one, who supplicates a lot; and I recommend you to supplicate specially at the time of dawn until sun-rise, because, at this time the gates of Heaven are opened; people's sustenance is distributed and their great wants are granted."
-al-Kafi vol. 2, p-478.
The Holy Prophet (S) has said:
"Supplication is believer's arsenal; is the pillar of religion and light of the earth and sky.”
-al-Kafi, vol. 2, p-468.
Supplication is a worship rather is the soul of all worships and brings eternal rewards. It is a believers ascension to Heavenly Kingdom, makes the supplicant's spirit perfected and nourished and helps suppliant to attain God’s Nearness. The Commander of the Faithful Imam' Ali (A) said:
"Supplication is the key of prosperity; the best supplication is the supplication which comes out .from pure chests and pious hearts; supplication or hymns with God-Almighty results in salvation; and through means of sincerity one is saved from adversities and wickedness, therefore, when hardships become intense one should seek refuge in God-Almighty."
-al-Kafi, vol. 2, p-468.
Therefore, supplication is a worship that if done properly in accordance with relevant conditions will result in suppliant's attaining self-perfection and God's Nearness, and these results certainly depends upon the rank and degree of the supplication. Because, of this reason a Lord's servant at any place, under any circumstances should never be negligent from this great worship, since a supplication is never going to be ineffective, although it may not produce immediate and apparent results. It is possible that sometimes the grant of suppliant's wants might be delayed; or may be they will not be fulfilled in this world; but even this is not without wisdom; because, occasionally a believer's demand are not in his real interest and a Wise God knows better what is really good for his servant.
Therefore, a servant should always stretch his hands before the Almighty Omnipotent God, and should supplicate for his wants. If it is deemed appropriate his wants will be granted in this world. But, sometimes God-Almighty thinks it appropriate to delay the grant of his servant so that he does more intensive humming communication. with Him, .thus, attaining higher exalted spiritual positions. Sometimes, divine expediency dictates that his grant should not be granted in this world so that he should remain continuously engaged in God's Remembrance, and receive a much better reward in the next world. The Holy Prophet (S) said:
"May God-Almighty bless the servant who seeks his needs from God-Almighty and pleads for their fulfillment through supplications whether his wants are granted or not. Then he recited the following verse:
"It may be that in prayer unto my Lord, I shall not be unblessed.”
-the Holy Quran (19:48)
-al-Kafi vol. 2 p-475.
Imam al-Sadiq (A) said:
"Sometimes a believer supplicates for a need before God-Almighty; but, He orders His Angels to delay the grant of servant's needs, because, He loves to hear his voice and supplication more. Then on the Day of Judgment says to him: ' Oh My servant! You called me but I delayed your request, now in return I will bestow upon you such and such reward and so and so supplication. ' Hearing it the servant would say: I wish none of my needs would have been granted in the world. ' He says so because he sees the excellent rewards of Hereafter."
-al-Kafi, vol. 2, p-490.
"Be careful about the etiquettes of supplication, and pay attention as to which personality are you talking, how do you beseech Him, and for what purpose is He implored?
"Think about the Majesty and Splendor of God-Almighty, and look inside your heart and know that He is aware of whatever is contained therein,. He knows about your heart' s secrets and the truth and falsehood hidden therein. Be careful, to identify correctly the path of your salvation or misfortune lest you request a thing from God-Almighty which contains your destruction while you imagine your salvation in it. The God-Almighty said in Holy Quran:
"Man Prayeth for evil as he prayeth for good, for man was ever hasty.”
-the Holy Quran (17:11)
"Therefore, think correctly regarding what do you want from God-Almighty and for what purpose is it required. A supplication will be accepted only if you exert absolute concentration of your entire existence towards God-Almighty,. melting your heart while witnessing His presence; abandoning all your disposals, and absolute surrender of all affairs with sincerity to God-Almighty. So, if you did not act in accordance to above mentioned conditions of supplication do not look forward for its acceptance.
"Because, God-Almighty is aware of all your secrets and mysteries. Perhaps you beseech God-Almighty for something, while you know that your intention is opposite to your request."
 Even if one need's an insignificant thing like shoe's laces, one must pray and ask God-Almighty [Tr].
 Al-Sahifah Al-Sajjadiyyah: includes certain supplications quoted from Imam Zain al-Abidin 'Ali b. Hussain b. 'Ali ibn Abi Talib (A). He is one of the Imams belonging to the household of the Prophet whom Allah has kept pure and free of defilement. The Imam was the fourth in line of the Imams of the Prophet's household. Imam' Ali ibn al-Husain (A), was born in the year 38 A.H. or, perhaps as is conjectured, a little before that and lived for a period of 57 years.
Imam al-Shafi considered Imam" Ali ibn al-Husain (A) as the most supreme jurist of all the people of Medina" Abd al-Malik bin Marwan said to him, "in the area of religious sciences, in devotion and piety, you have been granted that which no one before you has had other than your ancestors". Further Umar bin ' Abd al-Aziz said, "The light of this life, ' the beauty of Islam is Zain al-Abidin" Al-Sahifah Al-Sajjadiyyah represents and stands out as a profound social work of the time and a reflection of a supreme endeavor to meet the exigencies of spiritual ordeals facing the society at the time of the Imam. But beyond this it is a profound collection of supplications in the divine tradition, a unique compilation which will remain throughout the ages as a gift to mankind, a work of moral inspiration for worldly conduct and a torch of guidance. Human begins will constantly remain in need of this heavenly souvenir; and the need increases whenever Satan comes to increase the allurements of the world for people, and by its fascination to keep them in bondage [Tr].