It must be clearly understood, that regarding the friendship of believers, what really constitutes its axis, foundation, and criteria is - that to be friendly with them, should be only for Allah's sake and not for the sake of any other motive. It is this type of friendship of believers, which have been considered as precious and esteemed in the Islamic traditions and the Holy Qur'an, it has been emphasized a lot, so much so that in accordance with the following narration:
"It has been considered as the most stable pillar of belief."
And in another tradition:
"The faith has been defined, as having such sincere friendships with believers."
In praising the believers who love each other in this manner, a lot has been written in traditions. For example, it has been narrated in a tradition:
"Whenever the two friends shake their hands Allah too puts his hand between their hands and presses the hand of the one who is more steadfast and sincere in that friendship."
Regarding the believer who prays for other believers it has been narrated:
"Whenever a believer prays for a fellow brother believer, an angel from the worldly sky says to him: 'O' servant of Allah! Know that whatever you have asked for your fellow brother believer a thousand times of that would be given to you; the second angel makes triple and so on so forth until the seventh angel announces seven hundred thousands times of reward bestowed upon him. Then Allah announces glad tidings to him saying: 'O' My servant! My treasure never gets emptied through My donations! I will bestow upon you million times of whatever you have asked for your fellow brother believer.'"
Regarding the believers who help and support each other or fulfil the needs of other believers, there are plenty of traditions. It has been narrated in a tradition:
"Whoever fulfils the wants of a brother believer is like the one who has worshiped Allah with fasting during day and offering night prayer for nine thousands years."
It has been narrated in another tradition:
"Whoever does not mobilize all his resources and strength in
fulfilment of a brother believer's wants has indeed done a treachery towards Allah, His Prophet, and the believers."
The story of Imam al-Sadiq [a] and Hasan bin Yaqtin is very famous in this regard. In summary, Imam al-Sadiq write a brief recommendation letter in favour of Hasan bin Yaqtin to the ruler of Ahwaz as follows:
"Before Allah there is a heavenly shade or canopy in which no one can rest but without opening a knot of difficulties for a brother believer, or help him, or do some good however small it may be in his favour. And now a brother believer has approached you, regards.
The ruler of Ahwaz after reading this letter came out bare-footed to welcome Hasan, received him warmly, kissed his face, asked him about Imam's condition, then took him inside his home and treated him with warmth and affection, fulfilled his requirements, forgave his taxes, and divided equally all his wealth, horses, and slaves between him and Hasan.
"Of course, he undertook all these good actions and justified them for Allah's sake and in the path of Ahlul-Bait's [a] friendship . After a little while Hasan returned before Imam al-Sadiq [a] and described his encounter with the ruler of Ahwaz then the Imam said: 'By Allah, because of this kindness shown by the ruler towards you I am happy, so too my impeccable forefathers, the Commander of the Faithful Imam 'Ali [a], the Holy Prophet [s] and Allah."
Also, there are plenty of traditions about those who make believers uncomfortable. For example, Hussein bin Abi al-Ala has narrated:
"Once together with twenty or more pilgrims I went to Mecca and sacrificed a sheep at every lodging station for my fellow pilgrims. Later on when I had a chance of seeing Imam al-Sadiq [a], he said to me: 'woe be upon you Hussein! Why did you hurt the believers and make them feel insulted?' I said: 'I seek Allah's shelter from doing such a thing.' He said: 'I heard that you sacrificed a sheep at each lodging stations for your fellow companions.' I said: 'O' my master! By Allah, in this act I don't have any other intention except seeking Allah's pleasure.'
"He said: 'Did not you realize that perhaps among them there might have been others who would have liked to do the same thing, but since they do not have the financial strength to do such a thing, considered themselves as wretched and insulted?' I said: 'O' son of the Holy Prophet [s] salutation and greetings of Allah be upon you! I seek Allah's pardon, and would never indulge again in such an act.'"
Here, we would overlook from mentioning about plenty of fascinating traditions dealing with this matter, and limiting our discussion to only few above-mentioned narrations.