Is it obligatory to use all the water designated for ablution (wudu') that has been affected by the evil eye (al-ayn)? And is it permissible for another person to use the leftover water from that (wudu') for treatment from the same evil eye?

Being afflicted by the evil eye is real, and Allah has prescribed a cure for it through ritual washing, where the one who cast the evil eye (the 'ā'in) is commanded to perform ablution (wudu'), and then the one afflicted by the evil eye (the ma'yūn) washes with this water.

If the person afflicted by the evil eye washes with the ablution water of the 'ā'in, then the intended purpose has been achieved. There is no harm in using the remaining water for another person who suspects they have been afflicted by the evil eye by the same person.

The ablution water of the 'ā'in is requested when there are clear signs of affliction by the evil eye. However, merely suspicion and obsessive thoughts (waswasa) should not prompt this, as it can cause embarrassment and open the door to such obsessive thoughts.