Musannaf Ibn Abi Shayba 21:330

Ibn Numayr narrated to us, from Isma'il, from Abu Sinan, from Abu al-Ahwas, from 'Abdullah ibn Mas'ud, who said: "Whoever says, 'I seek forgiveness from Allah, besides whom there is no deity, the Ever-Living, the Sustainer, and I repent to Him,' three times, he will be forgiven, even if he had fled from the battlefield."