Musnad Abu Ya'la al-Mawsili 110:65

Abu Khaythama narrated to us, Ibn 'Uyaynah narrated to us, from 'Amr, who said: Abu Ma'bad mentioned it from Ibn 'Abbas, who said—then he denied it afterward from Ibn 'Abbas—who said: We used to know the conclusion of the prayer of the Messenger of Allah, may Allah bless him and grant him peace, by the *takbir*.