What is the ruling on one who, out of ignorance, washed his hand in ritual ablution (wudu') from the wrist to the elbow, then learned what was obligatory and started washing from the fingertips to the elbow? What is the ruling on his previous ablutions and prayers?

If one neglects washing the hands at the beginning of ablution, their ablution is invalid by agreement. If they were ignorant of this ruling, there is a difference of opinion regarding the necessity of repeating the prayers. However, if they wash their hands with the intention of it being obligatory or recommended, their ablution is valid according to the Hanafis and Malikis. It is hoped that you will not oblige them to repeat the prayers they have performed, though it would be good if they were to repeat them as a precaution.