(5) In arguing for the obligation of rubbing a part of the head...
All Shia scholars believe that rubbing a part of the head is obligatory and considered it necessary to rub the front part of the head. However, there is controversy in determining its amount. What is common among Shia jurisprudents is that naming (something) rubbing the front part of the head would suffice. (7) The proof for this famous statement is the valid traditions of the (a.s).
(8) We bring here one of these traditions narrated by Imam Baqir (a.s): Zurarah asked the Imam (a.s), “How do we know that rubbing a part of the feet is enough?” Imam Baqir (a.s) replied: O Zurarah! This is the word of the Messenger of Allah (a.s) and the same is true in what has been revealed in Allah’s Book; since Allah states, “wash your faces;” so, He told us to wash the whole face.
Then He said, “and your hands as far as the elbows.” Then He distinguished these two proceeding statements by saying, “Rub your heads.” When He said, “your heads,” He taught us that rubbing will be done with part of the head, as He had attached the ruling of the hands to that of the face, so He stated, “and your feet to the ankles.” Thus, by attaching the ruling of the feet to that of the head, He taught us that rubbing a part of the feet is sufficient.
As is evident from this tradition, Imam Baqir (a.s) has adduced both the Prophet’s tradition and the Quran’s surface and lexical aspect. To Shia jurisprudents, rubbing the head is recommended to be horizontally and with three closed fingers in amount. Rubbing or Washing the Feet? The most controversial issue in the ritual ablution among the Sunni and Shia is what to do about the feet.