Purity: Validity of Wudu Performed Before Prayer Time
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 320:
If someone is certain that the time for prayers has set in and makes the intention of performing an obligatory wuḍūʾ, and after performing wuḍūʾ he realises that the time has not set in yet, the wuḍūʾ is valid.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)