Musnad Abu Ya'la al-Mawsili 42:366

Zuhayr narrated to us, Muhammad ibn Khazim narrated to us, Ubayd Allah ibn al-Walid al-Wassafi narrated to us, from Atiyyah al-Awfi, from Abu Sa’id al-Khudri, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever says when he goes to his bed: 'I seek forgiveness from Allah, besides whom there is no god, the Ever-Living, the Sustainer, and I repent to Him' three times, Allah will forgive his sins, even if they were like the foam of the sea."