This is another article in the chapter Enormities from the classical manual on Sharia called Reliance of the Traveller. The articles are posted one by one in the order they are given in the book.
p47.0 ATTACKING ANOTHER’S ANCESTRY
p47.1 The Prophet (Allah bless him and give him peace) said,
“Two qualities in people are unbelief: attacking another’s ancestry, and wailing over the dead.”
(N: The hadith does not mean that these things put one beyond the pale of Islam, but that they are the actions of the unbelievers.)
A perfect example of Islam’s dualism: Muhammad was attacking the Meccans’ ancestry all the time he preached Islam, condemning their forefathers to hell for being pagans.