Sahih Al Bukhari – Book of Belief

Volumn 001, Book 002, Hadith Number 045. 
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Narated By Abu Huraira

said, "(A believer) who accompanies the funeral procession of a Muslim out of sincere faith and hoping to attain Allah's reward and remains with it till the funeral prayer is offered and the burial ceremonies are over, he will return with a reward of two Qirats. Each Qirat is like the size of the (Mount) Uhud. He who offers the funeral prayer only and returns before the burial, will return with the reward of one Qirat only."