Musannaf Ibn Abi Shayba 3:2245

Ibn Idris narrated to us, from Muhammad ibn Ishaq, from Ya'qub ibn 'Abd Allah al-Ashaj, from Bishr ibn Sa'id, from al-Harith ibn al-Mukhallad, from his father, who said: 'Umar said: "Whoever raises his head before the Imam, let him repeat [the action], and let him remain [in that position] until he sees that he has caught up with what he missed."