Purity: Wudu with Forgotten Usurped Water
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 271:
If a person forgets that some water is usurped and performs wuḍūʾ with it, his wuḍūʾ is valid. However, if someone usurps some water himself and then having forgotten that it is usurped performs wuḍūʾ with it, then to consider his wuḍūʾ valid is problematic [i.e. based on obligatory precaution, it must not be considered valid].
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)