Sahih Bukhari Volume 001, Book 002, Hadith Number 046.
Narated By 'Abdullah : 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)."