Musannaf Ibn Abi Shayba 28:534

Ya'la ibn 'Ubayd narrated to us, from al-Ajalih, from 'Ammar al-Duhni, from Abu al-Tufayl, who said: 'Ali sent Ma'qal al-Taymi to the tribe of Najiya and said: "When you come to the people, call them [to Islam] three times."