Musannaf Ibn Abi Shayba 21:564

Ibn ‘Ulayyah narrated to us, from Salamah ibn ‘Alqamah, from Muhammad ibn Kathir ibn Aflah, who said: I prayed. He said: When it was the final sitting, I did like this—Ibn ‘Ulayyah gestured with his two fingers—so Ibn ‘Umar grasped this, meaning the left one.