Musannaf Ibn Abi Shayba 13:1884

Abu Bakr narrated to us, he said: Ibn ‘Uyaynah narrated to us, from ‘Amr ibn Dinar, who said: ‘Umar ibn al-Khattab passed by a scribe who was writing between people, and he was witnessing [the transaction] with more than two [witnesses], so he forbade him. Then he passed by him afterward and said, "Did I not forbid you?" The man said, "I obeyed Allah and disobeyed you." And in the charity of ‘Umar, ‘Abdullah ibn al-Arqam and Mu‘ayqib witnessed. And in the charity of ‘Ali, so-and-so and so-and-so witnessed.