Musnad Ahmad 31:87

Abu Mu'awiyah 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, the One 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, even if they were like the sand of 'Alij, and even if they were like the number of the leaves of the trees."