(30) باب إِطْلاَقِ اسْمِ الْكُفْرِ عَلَى الطَّعْنِ فِي النَّسَبِ وَالنِّيَاحَةِ عَلَى الْمَيِّتِ
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.
Reference: Sahih Muslim 67
In-book reference: Book 1, Hadith 133
USC-MSA web (English) reference(deprecated numbering scheme): Book 1, Hadith 128