Reference: Sharh ‘Umdatil-Fiqh – Tape No.9, Question No.47
Question: Whilst in mid-prayer, if he remembered [not having made] the intention, is it sufficient that he then makes the intention whilst he is praying?
Response: How is [it then possible] that he made the ablution and prayed? This in itself is proof as to the presence of the intention. It is not possible that he was afflicted with whispers [from Shatytaan], [since] he got up [and prepared himself for prayer] and [then] came for the prayer and pronounced “Allaahu Akbar” – all of this [here] is proof as to the presence of the intention.