Musnad Abu Ya'la al-Mawsili 152:209

Abu Khaythama narrated to us, Abd al-Rahman narrated to us, Isra'il narrated to us, from Abu Ishaq, from 'Amr ibn Maymun, from Abdullah ibn Mas'ud, who said: The Messenger of Allah (may Allah bless him and grant him peace) used to like that we supplicate three times and seek forgiveness three times.