Ruling on standing for a fifth rak'ah by mistake in a four-rak'ah prayer

Question:
I led the prayer in the Zuhr prayer, and in the fourth rak'ah, I became uncertain whether I had performed the fourth or not. I stood up to perform the fourth, and the followers of the prayer recited the tasbih. Naturally, I sat down for the tashahhud. Do I have to perform the prostration for forgetfulness, and when should I prostrate?

Answer:
If one is uncertain in the fourth rak'ah whether it is the fourth or the third, and he stands to perform the fourth, he should be alerted; for he should sit, recite the tashahhud, complete the prayer, and then prostrate for forgetfulness before the taslim. Because at that point, he built his prayer upon certainty when he stood, intending to build upon certainty. The Messenger of Allah (peace and blessings be upon him) ordered the one who builds upon certainty to prostrate for forgetfulness before the taslim. The Sunnah is to prostrate before the taslim.
As for the one who missed a rak'ah, or missed two rak'ahs, for example, it is better for him to prostrate after the taslim. He completes his prayer when he is alerted, then after he recites the tashahhud and the supplication, he performs the taslim, and then he prostrates for forgetfulness two prostrations, as the Prophet (peace and blessings be upon him) did in the story of the man with the two hands when he performed the taslim after two rak'ahs, and the hadith of 'Imran ibn Husayn when he performed the taslim after three, he completed his prayer and performed the taslim from it, then prostrated for forgetfulness after that.