Musannaf Ibn Abi Shayba 3:2880

Yazid ibn Harun narrated to us, he said: Sa'id ibn Abi 'Aruba narrated to us, from Sa'id ibn Abi Harra, from Nafi', who said: Ibn 'Umar used to sit with his knees drawn up to his chest (ihtiba') on Friday while the Imam was delivering the sermon. If it became long, he would place his head in his lap.