Purity: Certainty or Doubt About Missed Parts During Wudu
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 302:
If a person becomes certain after or while performing wuḍūʾ that he has not washed or wiped some necessary places, in the event that the moisture on the previous parts has dried up due to the passing of time, he must perform wuḍūʾ again. If the moisture has not dried up, or it has dried up on account of hot weather and suchlike, he must wash or wipe the part that he forgot and the parts that follow it. If during wuḍūʾ one doubts whether or not he has washed or wiped a place, he must follow the same instructions.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)