Musannaf Ibn Abi Shayba 33:854

'Affan narrated to us, he said: Bukayr ibn Abi al-Sumayt narrated to us, he said: Mansur ibn Zadhan narrated to us, from Abu al-Siddiq al-Naji, from Abu Sa'id al-Khudri, he said: Whoever says, "I seek forgiveness from Allah, the One 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 his sins were like the foam of the sea.