Does abandoning a wajib intentionally or unintentionally in a prayer harm that prayer? What has to be done in that case?

Dear Brother / Sister,

According to Hanafis, fard and wajib have different meanings.  Wajib is a deed that Allah and His Messenger want bindingly a responsible Muslim to do but this bindingness is definite based on uncertain evidence. Accordingly, the certainty of wajib is less than fard.

Therefore, if fard is abandoned in a religious deed, this deed becomes invalid.  For instance, the hajj of a person who does not perform waqfa in Arafat becomes invalid because waqfa is fard. However, if a person abandons sa'y between Safa and Marwa, his hajj does not become invalid. It is possible to complete it by sacrificing an animal.

Abandoning reading the Quran completely in prayer makes a prayer invalid. For reading some part of the Quran in prayer is based on certain evidence. The following is stated in a verse:

"Read ye, therefore, of the Qur'an as much as may be easy for you."(al-Muzzammil, 73/20).

However, not reading the chapter of al-Fatiha is not a reason that invalidates a prayer on its own. If this chapter is not read, the prayer is makruh but valid. 

A person who abandons a wajib by mistake or who forgets a wajib needs to perform sajdah as-sahw at the end of the prayer.

It is makruh tahrimi to abandon one of the wajibs of prayer, for instance reading the chapter of al-Fatiha, deliberately and intentionally. This prayer, which becomes makruh tahrimi because abandoning a wajib deliberately, is essentially valid and the person is not regarded responsible for it but it is wajib to perform it again.

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