Sahih Bukhari Volume 002, Book 013, Hadith Number 049.
Narated By Ibn Abbas : Once the Prophet ascended the pulpit and it was the last gathering in which he took part. He was covering his shoulder with a big cloak and binding his head with an oily bandage. He glorified and praised Allah and said, "O people! Come to me." So the people came and gathered around him and he then said, "Amma ba'du." "From now onward the Ansar will decrease and other people will increase. So anybody who becomes a ruler of the followers of Muhammad and has the power to harm or benefit people then he should accept the good from the benevolent amongst them (Ansar) and overlook the faults of their wrong-doers."