Musannaf Ibn Abi Shayba 3:2271

Abu Khalid al-Ahmar narrated to us, from Isma'il, from 'Amr ibn Dinar, who said: I saw 'Ubayd ibn 'Umayr praying while in the *ihtiba'* position (sitting on his buttocks with his knees drawn up to his chest, held by his arms or a cloth).