What is the ruling on someone who performed ablution (wudu') and then prayed, but doubted during the prayer whether they had washed one of their limbs? Should they interrupt their prayer? And what is the ruling if they doubt after the prayer, and should they repeat it? And what is the fundamental principle – certainty (yaqeen) – in this situation?

Doubt about washing a limb during ablution, after having finished it, is not to be heeded. One must disregard it and continue with the prayer, because doubt concerning an act of worship after its completion and conclusion is not to be heeded by the scholars. This is not like doubt during the act of worship, in which case the one who doubts builds upon certainty.