What one Says in the Morning, the Evening and when going to Sleep - Section 2 — Mishkat al-Masabih 2398

Anas reported God's messenger as saying that if anyone says in the morning, “O God, in the morning we call Thee, the bearers of Thy Throne, Thy angels and all Thy creatures to witness that Thou art God than whom alone there is no god, being without a partner, and that Muhammad is Thy servant and messenger,” God will forgive him any sins he commits that day; and if he says it in the evening, God will forgive him any sins he commits that night. Tirmidhi and Abu Dawud transmitted it, Tirmidhi saying this is a gharib tradition.