Musnad Ahmad 27:1888

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from 'Ubayd Allah ibn 'Umar, from Nafi', from Ibn 'Umar: "That the Messenger of Allah, may Allah bless him and grant him peace, when he sat in the prayer, he would place his hands upon his knees, and he would raise his right finger which is next to the thumb, and he would supplicate with it. And his left hand was upon his knee, spreading it out upon it."