Musannaf Ibn Abi Shaybah

Hazrat Mujahid narrates that Hazrat Ibn Umar (may Allah be pleased with them) would, after the break of dawn, perform the Sunnah prayers of Fajr and then sit cross-legged. We would sit around him. When he saw any of us dozing off, he would shake him; he himself would also be dozing in the cross-legged position. Then, when the prayer was established, he would get up and join the congregation.