Sahih Muslim Book 4, Hadith Number 2072.

Chapter : If one hundred Muslim offer prayer for the dead, all of them interceding for him (it would he accepted).

‘Abdullah b. ‘Abbas reported that his son died in Qudaid or ‘Usfan. He said to Kuraib to see as to how many people had gathered there for his (funeral). He (Kuraib) said: So I went out and I informed him about the people who had gathered there. He (Ibn ‘Abbas) said: Do you think they are forty? He (Kuraib) said: Yes. Ibn ‘Abbas then said to them: Bring him (the dead body) out for I have heard Allah’s Messenger (may peace be upon him) as saying: If any Muslim dies and forty men who associate nothing with Allah stand over his prayer (they offer prayer over him), Allah will accept them as intercessors for him.