(1)
Ibn 'Abbas (RAA) narrated, 'I heard the Messenger of Allah (ﷺ) say, "If a Muslim man dies and a group of forty people, who do not associate any one with Allah, pray for him, Allah will accept their intercession for him (by way of their Du'A for him." Related by Muslim.
Reference: Bulugh al-Maram 558
In-book reference: Book 3, Hadith 26
English translation: Book 3, Hadith 582