Musannaf Ibn Abi Shayba 3:567

Ibn Numayr narrated to us, from al-A’mash, who said: I saw ‘Atiyyah sitting on his heels (iq’a) between the two prostrations, so I said to him: He said: I saw Ibn ‘Umar, Ibn ‘Abbas, and Ibn al-Zubayr sitting on their heels (iq’a) between the two prostrations.