Musannaf Ibn Abi Shayba 3:1100

Affan narrated to us, he said: Abu Hilal narrated to us, he said: Muhammad ibn Sirin narrated to us, he said: Umar ibn al-Khattab used to fear forgetfulness. He said: So when he would pray, he would appoint a man, and he would observe him. If he saw him stand, he would stand, and if he saw him sit, he would sit.