It is narrated about Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both). One of his sons passed away at Qudayd, or perhaps at ‘Usfan, so he said: “O Kurayb, check how many people have gathered for his funeral prayer.” The narrator says: I went outside and found that some people had gathered, so I informed Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) about it. He asked: “Are there forty people?” I replied: “Yes.” So Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said: “Bring out his body, for I heard the Noble Prophet (peace and blessings be upon him) say: ‘Whenever a Muslim passes away and forty people who do not associate anything with Allah perform his funeral prayer, Allah accepts their intercession for that deceased.’”
Hadith Referenceصحیح ابن حبان / كتاب الجنائز وما يتعلق بها مقدما أو مؤخرا / 3082
Hadith Gradingفضيلة الشيخ الإمام محمد ناصر الدين الألبانيصحيح - «الأحكام» (127).فضيلة الشيخ العلّامة شُعيب الأرناؤوطإسناده حسن على شرط مسلم