What is the ruling on one who omits a pillar of prayer, whether forgetfully or intentionally?

If one deliberately omits a pillar of prayer, their prayer becomes void. If one omits it out of forgetfulness and a long time passes, it becomes void. If a short time passes, it does not become void, and according to Imam Ahmad, one must perform a complete rak'ah. According to Imam Al-Shafi'i, one must perform the pillar and what comes after it, and this is the more preponderant view.

If one remembers it during the prayer, they should perform it and what comes after it, and then perform the prostration of forgetfulness (sujud al-sahw). If they do not remember it until it is too late to rectify it by reaching its place in the subsequent rak'ah, the rak'ah in which the pillar was forgotten becomes void, and the subsequent rak'ah takes its place. The prayer is not valid if the takbirat al-ihram is omitted. Standing is waived in supererogatory prayers.