Sahih al Bukhari 47

Narrated Abu Huraira: Allah's Messenger (ﷺ) 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

Sahih al Bukhari 47
Book 2, Hadith 40