Musnad Ahmad 27:594

Muhammad ibn Ja‘far and Hajjaj—the meaning is the same—said: Shu‘bah narrated to us, Muslim ibn Abi Maryam narrated to us—Hajjaj said: from the Banu Umayyah—he said: I heard ‘Ali ibn ‘Abd al-Rahman—Hajjaj said: the Umayyad—he said: I heard Ibn ‘Umar, and he saw a man fidgeting in his prayer. Ibn ‘Umar said: "Do not fidget in your prayer, and do as the Messenger of Allah, may Allah bless him and grant him peace, used to do." Muhammad said: "Ibn ‘Umar placed his right thigh upon his left, and his left hand upon his left knee, and he placed his right hand upon his right [thigh], and he gestured with his finger."