Imam As-Sadiq (a.s.) said:
اَلْعُبّادُ ثَلثَةٌ: قَوْمٌ عَبَدُوا اللّهَ خَوْفاً فَتِلْكَ عِبادَةُ الْعَبِيد، و قَوْمٌ عَبَدُوا اللّه طَلَبَ الثَّوابِ فَتِلْكَ عِبادَةُ الأُجراءِ، وَ قَوْمٌ عَبَدُوا اللّهَ حُبًّا لَهُ فَتِلْكَ عِبادَةُ الأَحْرارِ.
Worshippers fall into three groups:
Those who worship Allah for the fear of hell; this is the worship of slaves.
Those who worship Allah for his reward; this is the worship of mercenaries.
Those who worship Allah out of love and affection for Him; this is the worship of free men.1
Although the promises of divine reward and punishment are all true, and his reward is very valuable and his chastisement is very painful, there are some high minded free men who do not see anything save God, and do not seek anything save God, and the measure of their heart is full of love and kindness. They are looking beyond rewards and punishment; their motive for observing God’s command is only love mingled with his cognition and knowing.