Purity: Doubt About Purifying an Impure Part
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 277:
If one of the parts of the body on which wuḍūʾ is performed is impure, and after performing wuḍūʾ one doubts whether or not he washed it with water before performing wuḍūʾ, the wuḍūʾ is valid. However, he must still wash the part that was impure with water.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)