Musannaf Ibn Abi Shayba 3:2866

Al-Dahhak ibn Makhlad narrated to us, from Salim al-Khayyat, who said: I saw al-Hasan, Muhammad, Ikrimah ibn Khalid al-Makhzumi, Amr ibn Dinar, Abu al-Zubayr, and Ata sitting with their knees drawn up to their chests (ihtiba') on Friday while the Imam was delivering the sermon.