Sahih Bukhari Volume 2, Book 23, Hadith Number 405.

Narated By Ash-Shaibani : Ash Sha’bi said, “I was informed by a man who had seen the Prophet going to a grave that was separate from the other graves and he aligned the people in rows and said four Takbir.” I said, “O Abu ‘Amr! who narrated (that) to you”? He said, “Ibn Abbas.”