Musannaf Ibn Abi Shayba 3:4059

Hushaym narrated to us, he said: Husayn informed us, from Mujahid, who said: When the dawn would break, Ibn Umar would pray two rak'ahs, then he would sit in the iḥtibā' position (sitting with his knees drawn up to his chest, held by his arms or a cloth) while we were around him. If he saw anyone among us becoming drowsy, he would move him. He said: And he would become drowsy while he was in the iḥtibā' position. Then the prayer would be established, so he would rise and pray.