Purity: Body Part Becoming Impure During Wudu

Practical Laws of Islam as per the teachings of Ayatullah Sistani

Ruling 275:

If a place that has been washed or wiped becomes impure before the wuḍūʾ has been completely performed, the wuḍūʾ is valid.

-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)