Musnad Ahmad 26:197

Yahya ibn Adam narrated to us, Isra'il informed us, and Abu Ahmad narrated to us, Isra'il narrated to us, from Abu Ishaq, from 'Amr ibn Maymun, from 'Abdullah—Abu Ahmad said: from Ibn Mas'ud—who said: "The Prophet, peace and blessings of Allah be upon him, used to like to supplicate three times and seek forgiveness three times."