What is the ruling on one who omits a Sunnah, an obligatory act (wajib), or a pillar (rukn) in ablution or prayer, whether the omission was due to denial, forgetfulness, or intention?

Ablution (wudu') is a prerequisite for the validity of prayer (salat). Scholars divide the acts of ablution into obligatory (fara'id) and sunnah (recommended) acts. The obligatory acts are: intention, washing the face, washing the hands up to the elbows, wiping the head, washing the feet up to the ankles, observing the sequence, and maintaining continuity. If a Muslim intentionally or unintentionally omits any of these, their prayer is invalid and must be repeated. However, omitting sunnah acts does not affect the validity of ablution or prayer. Therefore, the prayer of someone who did not wash their feet is invalid. As for one who intentionally omits a pillar (rukn) of prayer, their prayer is invalid. According to some scholars, like the Hanbalis, prayer becomes invalid by intentionally omitting a waajib (obligatory act) but not by unintentionally omitting it.