Hadith 6:644
Ibn Fudayl narrated to us, from al-‘Ala’, from ‘Amr ibn Murrah, who said: ‘Umar said, “Everyone has done it.” So they said, “Let us agree upon a matter that those who come after us will adopt.” Thus, they performed four takbirs over the funeral prayer.
حدثنا بن فضيل عن العلاء عن عمرو بن مرة قال قال عمر كل قد فعل فقالوا نجتمع على أمر يأخذ به من بعدنا فكبروا على الجنازة أربعا