What should one do who forgot to sit for the first Tashahhud in the second rak'ah and remembered this in the third rak'ah: should he sit then complete his prayer, or should he complete his prayer with a single sitting, and does he have to repeat the prayer or perform the prostration of forgetfulness (sujud al-sahw)?

If the Imam or an individual praying alone, out of forgetfulness, stands up for the third rak'ah in a three- or four-rak'ah prayer without sitting for the first tashahhud, and does not remember until he has fully stood up, he should continue his prayer, complete it, and perform the two prostrations of forgetfulness (sujud al-sahw) before the salutation (tasleem). He should not perform tashahhud after the third rak'ah of a four-rak'ah prayer. The first tashahhud is waived for him, and sujud al-sahw compensates for it. He is not required to repeat the prayer if a long time has passed and he did not perform sujud al-sahw.