Musannaf Ibn Abi Shayba 21:327

'Affan narrated to us, [he said] Bukayr bin Abi al-Sumayt narrated to us, [he said] Mansur bin Zadhan narrated to us, from Abu al-Siddiq al-Naji, from Abu Sa'id al-Khudri, who said: Whoever says, "I seek forgiveness from Allah, the Magnificent, whom there is no god but He, the Ever-Living, the Sustainer, and I repent to Him," five times, he will be forgiven, even if he had [sins] like the foam of the sea.