What is the ruling on the prayer of someone who forgot a post-prostration of forgetfulness (sujud al-sahw al-ba'diyyah) and remembered it after a long interval; does he have to repeat his prayer?

As long as you performed the prostration of forgetfulness (Sujud as-Sahw), even if after some time, we hope that you are not required to repeat the prayer. The fatwa adopted by us is that a long interval does not matter. However, if you were to repeat the prayer as a precautionary measure, it would be more absolving for your conscience. The Malikis hold that the prayer becomes invalid due to forgetting the pre-salaam prostration (Sujud Qabli) with a long interval, if what was forgotten was the sitting for the first tashahhud.

It states in Al-Kharashi's commentary on Mukhtasar Khalil that the prayer becomes invalid by omitting the pre-salaam prostration of forgetfulness if it was due to the omission of three sunnah acts and there was a long interval, such as omitting the sitting other than the last one. "Long" is determined by custom or by leaving the mosque.

If you suffer from obsessive doubts (waswasah) and frequent uncertainties, do not pay attention to your doubts.