Musannaf Ibn Abi Shayba 21:329

Mu'awiyah ibn Hisham narrated to us, Sharik narrated to us, from Abu Ishaq, from Mu'adh ibn Jabal, who said: "Whoever says, 'I seek forgiveness from Allah, besides whom there is no god, the Ever-Living, the Sustainer, and I repent to Him,' three times, he will be forgiven, even if he had fled from the battlefield."