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Imam Sadigh (a.s.) said:
worshippers are three groups: those who worship Allah for the fear from hell; this is the worship of slaves.
those who worship allah for his reward; this is the worship of mercenaries.
and those who worship Allah for his love and affection; this is the worship of freemen.

breif discription:
although the promises to divine reward and punishment are all true, his reward is very valuable and his chastisement is very painful, but those high minded freemen 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 for something higher than reward and punishment, and their motive for observing god's command, is only a love mingled with his cognition and knowing.

قال الصادق علیه السلام

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 عِبادَةُالأَحْرارِ

امام صادق علیه السلام می فرماید

عبادت کنندگان سه دسته اند: عده ای از ترس جهنم عبادت خدا را می کنند که آن عبادت بردگان است. عده ای برای کسب ثواب بندگی خدا را می کنند که عبادت تاجران است. و عده ای از روی شوق و علاقه، بندگی خدا را می کنند که آن عبادت آزادگان است