What should a person do when he forgets or doubts whether or not he is in the state of ablution?
If a person is sure that he performed ablution but he doubts if his ablution is invalidated, he is deemed as ritually clean. On the other hand, a person who knows for sure that his ablution is invalidated and doubts if he performed ablution afterwards, he is deemed as ritually unclean. Because a certain information cannot be eliminated by doubt. (Mawsili, al-Ikhtiyar, I, 56; Ibn Abidin, Radd al-Muhtar, I, 283).
Source: Presidency Of Religious Affairs The Turkey, High Board of Religious Affairs FATWAS
May One Wipe (Apply Masah) Over Headscarf While Performing Minor Ablution?
