Musannaf Ibn Abi Shayba 3:2270

Waki’ narrated to us, from Sufyan, from Ibn Abi Dhi’b, from al-Zuhri, from Sa’id ibn al-Musayyib, that he used to pray while in the *ihtiba* position (sitting on his buttocks with his knees drawn up to his chest, held by his arms or a cloth).