Sahih Bukhari Volume 2, Book 23, Hadith Number 408.

Narated By Ash-Shaibani : Ash-Sha’bi said, “Somebody who passed along with your Prophet (p.b.u.h) by a grave that was separate from the other graves informed me (saying), “The Prophet led us (in the prayer) and we aligned behind him.” We said, “O Abu ‘Amr! Who told you this narration?” He replied, “Ibn Abbas.”