Sahih Bukhari Volume 001, Book 008, Hadith Number 438.
Narated By 'Ikrima : Ibn 'Abbas said to me and to his son 'Ali, "Go to Abu Sa'id and listen to what he narrates." So we went and found him in a garden looking after it. He picked up his Rida', wore it and sat down and started narrating till the topic of the construction of the mosque reached. He said, "We were carrying one adobe at a time while 'Ammar was carrying two. The Prophet saw him and started removing the dust from his body and said, "May Allah be Merciful to 'Ammar. He will be inviting them (i.e. his murderers, the rebellious group) to Paradise and they will invite him to Hell-fire." 'Ammar said, "I seek refuge with Allah from affliction."