Musnad Ahmad 27:1693

'Affan narrated to us, Hammad ibn Salamah narrated to us, Ayyub informed us, from Nafi', from Abdullah ibn Umar: "That the Prophet, may Allah bless him and grant him peace, when he sat to recite the Tashahhud, would place his left hand on his left knee, and place his right hand on his right knee, and form the number fifty-three, and supplicate."