Sayyidah Maymunah (may Allah be pleased with her), whose foster brother was Abdullah bin Salit, narrated from the Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) said: "There is no Muslim over whom a congregation offers the funeral prayer but that their intercession for him is accepted."
Abu al-Malih used to say: By a congregation is meant forty people or more.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20362