Purity: Timing and Intention of Wudu
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 319:
At whatever time a person performs wuḍūʾ with the intention of attaining proximity to Allah, it is valid; and it does not matter whether he performs it shortly before the time for prayers, or well in advance of it, or after it has set in. Furthermore, it is not necessary for one to make the intention of performing an obligatory or recommended wuḍūʾ. In fact, even if one mistakenly makes the intention of performing an obligatory wuḍūʾ and afterwards realises that it was not obligatory, his wuḍūʾ is valid.
-Ayatullah Sistani, Practical Laws of Islam, Purification (Taharah)