Sahih Muslim
The Book of Faith (Kitab Al-Iman)
Book Number: 001 / Hadith Number: 128
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.