Musannaf Ibn Abi Shaybah

Hazrat Ibn Umar (may Allah be pleased with them both) used to sit in the posture of "Habwah" during the Imam's sermon. (The meaning of making "Habwah" is that a person sits on his buttocks, raises his knees, and ties both hands around them for support, or ties a cloth around his back and knees. The people of Arabia often used to sit in this manner.)