Abusing a Muslim is Fusuq and killing him is Kufr

Hazrat Abdullah narrated that:

 The Prophet said,

 "Abusing a Muslim is Fusuq [an evil doin] 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 1st,3rd, 5th, the 7th and the 9th [of the last 10 nights of the month of Ramadan]."