The World Federation One Stop Fiqh

Ritual 298

If a pilgrim is unaware of any najasah on his body or clothes, and he becomes aware of it only after completing tawaf, it is valid and there is no need to repeat it. Similarly, even the tawaf prayer will be valid, if he was unaware of the uncleanliness until after prayer.

The exception being, if he was in doubt about the najasah, before the prayer,or made investigation and did not become aware of it. However, the person who has a doubt, but does not carry out an investigation, and notices the uncleanliness after prayer, must repeat the prayer, as a matter of obligatory precaution.