Musannaf Ibn Abi Shayba 3:4019

Waki’ narrated to us, from his father, from Mansur, from Ibrahim, from Al-Aswad ibn Yazid, that he used to: When he prayed the two rak'ahs of the dawn (Fajr), he would sit in the iḥtibā' position (sitting on his buttocks with his knees drawn up to his chest, held by his arms).