Musannaf Ibn Abi Shayba 3:2269

Waki' narrated to us, he said: Sufyan narrated to us, from al-Hasan ibn 'Amr, from his father, he said: I saw Sa'id ibn Jubayr praying while in the *ihtiba* position (sitting with his knees drawn up to his chest and held by his arms or a cloth). When he wanted to bow, he would undo his *ihtiba*, then stand up and bow.