Sahih Muslim Book 1, Hadith Number 128.

Chapter : The use of the word “Unbelief” in case of slandering (anyone) or one’s lineage, and lamentation.

It is narrated on the authority of Abu Huraira that the Messenger of Allah (may peace and blessings be upon him) observed: Two (things) are found among men which are tantamount to unbelief: slandering one’s lineage and lamentation on the dead.