Musannaf Ibn Abi Shayba 3:6055

Ibn ‘Ulayyah narrated to us, from Salamah ibn ‘Alqamah, from Muhammad ibn Sirin, from Kathir ibn Aflah, who said: I prayed, and when I was in the final sitting, I did like this—and Ibn ‘Ulayyah gestured with his two fingers. Ibn ‘Ulayyah clenched these—meaning the left [hand].