Bukhari :: Book 1 :: Volume 2 :: Hadith 45
Narrated Abu Huraira:
Allah's Apostle 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."
Bukhari :: Book 1 :: Volume 2 :: Hadith 46
The Prophet said, "Abusing a Muslim is Fusuq (an evil doing) and killing him is Kufr (disbelief)." Narrated 'Ubada bin As-Samit: "Allah's Apostle went out to inform the people about the (date of the) night of decree (Al-Qadr) but there happened a quarrel between two Muslim men. The Prophet said, "I came out to inform you about (the date of) the night of Al-Qadr, but as so and so and so and so quarrelled, its knowledge was taken away (I forgot it) and maybe it was better for you. Now look for it in the 7th, the 9th and the 5th (of the last 10 nights of the month of Ramadan)."