If nothing is stronger in his mind, he should act on certainty, which is the lesser number, and prostrate for forgetfulness before the closing salam.

The Shaykh : As for when one says, "I have no preference in my mind," we say: act on certainty. So if you are unsure whether you prayed three or four units, make it three, because that is what you are certain of. Then complete the prayer for him, and then prostrate two times before the closing salam. So, if the doubt involves a stronger indication, we act on the stronger position and prostrate after the closing salam. And if there is no stronger indication, we act on certainty and prostrate before the closing salam. The wisdom behind this is apparent, because the doubt that involves hesitation without a stronger indication is a deficiency, so it is wise to make up for this deficiency before the closing salam. As for the doubt that involves a stronger indication, the weaker position is a mistake and has no effect, and the prostration is after the closing salam so that two additions do not occur in the prayer: an addition based on the stronger indication and an addition by the prostration. This is the summary regarding the prostration for forgetfulness.