Musnad Ahmad 27:911

'Affan narrated to us, Aban ibn Yazid narrated to us, Qatadah narrated to us, 'Abdullah ibn Babi al-Makki narrated to me, he said: I prayed beside 'Abdullah ibn 'Umar. He said: When he finished the prayer, he struck his thigh with his hand and said: "Shall I not teach you the greeting of the prayer just as the Messenger of Allah, may Allah bless him and grant him peace, used to teach us?" Then he recited these words to me, meaning the statement of Abu Musa al-Ash'ari in the Tashahhud.