Does a person who has donated blood have to perform ablution (wudu`) for the prayer? Or is his wudu` not nullified by donating blood?

All praise is for Allah, and may peace and blessing be upon His noble messenger.

To proceed: Imam Abu Hanifa and Imam Ahmad are both of the opinion that excessive amounts of blood nullify ablution, but Imam ash-Shafi`i and Imam Malik are of the opinion that it does not. The more correct—in my opinion—is to err on the safe side in matters of worship and to avoid the difference of opinion. It would be best for him to perform ablution again; and Allah knows best.

Dr. Muwaffaq al-Ghalayini