Answered by Ustadh Tabraze Azam
Is my wudu not valid if used water has dripped from my face onto my hands while I was washing the face three times?
Answer: Wa alaikum assalam wa rahmatullah,
No, if used water drips into your hands which contain pure and purifying water, your washing and ritual ablution (wudu) would remain valid. This is because used water is deemed pure, yet not purifying, and thus admixing with pure and purifying water is of no legal consequence.
The exception is when the volume of used water is greater than the volume of pure and purifying water, in which case, you should use a new handful of water. However, this is simply a scenario, and not really indicative of something you will ever experience.
In any case, we strive to avoid such drops falling into the water we are about to use as it is generally unbecoming, namely, due to such water having being used to lift the state of ritual impurity (hadath).
[Shurunbulali, Maraqi al-Falah (1.52-53)]
Please also see: Purification Issues: Washing Limbs & Used Water
And Allah Most High alone knows best.
Checked & Approved by Shaykh Faraz Rabbani