What is the Amount of the Head that is Fard to be Rubbed with Wet Hand?

The amount that is fard to be rubbed with wet hand is one-fourth of the head. The Messenger of Allah rubbed the part over the forehead, that is, the front part of the head. Therefore, it is sunnah to rub the front part of the head. However, any part of the head can be rubbed unless it is at least one fourth of the head and unless the part below the ears are rubbed. The place for masah is the part over the ears. If one fourth part of the head is rubbed at any part of the head, masah is regarded to be performed. 

According to Malikis and Hanbalis, it is fard to rub the whole head. According to Shafiis, to rub at least one inch of the head is enough.