What is the ruling on forgetting the intention (niyyah) in prayer and other acts of worship, knowing that it is one of the pillars, and not remembering it until after the completion of the worship?

Intention is one of the conditions for the validity of prayer, by scholarly consensus. Therefore, whoever prays without recalling the intention, their prayer is invalid and must be repeated. However, if the matter is merely a whisper (waswasa), it should not be heeded, because the worshipper's going to the mosque, awaiting the prayer, and entering into it indicates the presence of intention. If the meaning is not feeling complete sincerity, this diminishes the reward of the prayer, but it is an additional matter beyond the obligatory intention.