What is the ruling on doubting having concluded a supererogatory prayer after commencing an obligatory prayer; does it invalidate the obligatory prayer, or is only the supererogatory prayer to be repeated?

If one doubts whether he has made the taslim (definition) from his prayer, the original ruling is that he has not. If he then commits an invalidating act, such as deviating from the Qibla (definition), or if a long time passes, or if he speaks excessively, or makes an invalidating movement, his prayer becomes void. If he begins a second prayer after the first one has become void, the second prayer is valid.

However, if he begins the second prayer before the first one has become void: 1. If his standing in the second prayer becomes long by common understanding, or if he bows and prostrates in it, both prayers become void (the first due to the long interval, and the second because he began it while still in the first). 2. If his standing in the second prayer does not become long, and he remembers that he did not make the taslim from the first, or he doubts shortly after and has not committed an invalidating act, he must return to a sitting position and make the taslim from the first prayer; otherwise, both will become void.

Imam Al-Shafi'i said: If one begins an obligatory prayer before making the taslim from a supererogatory prayer, and he remembers shortly thereafter, he should make the taslim from the supererogatory prayer, perform the prostration of forgetfulness (sujud (definition) al-sahw), and then begin the obligatory prayer. If his standing in the obligatory prayer becomes long, or if he bows and prostrates in it, both prayers become void, and he must begin the obligatory prayer anew. Similarly, if he makes a mistake in an obligatory prayer and then enters into a supererogatory prayer, if what he did in the supererogatory prayer was short, he should return to the obligatory prayer, complete it, and perform the prostration of forgetfulness. But if it was long, or if he performed a bow (ruku'), the obligatory prayer becomes void, and he must repeat it.

All of this applies when doubt is not frequent. However, if doubt becomes frequent, it should be disregarded.