Narrated 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)."

Volume 1, Book 2, Number 46 :sahih bukhari