What is the ruling on someone who swore by intention of the heart to abandon a sin, then forgot his oath and returned to it?

If an oath is merely a thought and an intention in the heart without utterance by the tongue, then it is an uncontracted oath, and one is not bound to do what he swore to abandon. However, if one pronounces an oath and then does what he swore to abandon out of forgetfulness, then nothing is incumbent upon him, and no expiation is required, for Allah Almighty says: "Our Lord, do not impose blame upon us if we forget or make a mistake." And a Muslim must repent from all sins immediately.