If a person forgets tashahhud, stands up, and remembers before going into rukūʿ that he has not performed it, he must sit down, say tashahhud, stand up again, recite everything that must be recited in that rakʿah, and complete the prayer. And based on recommended precaution, after completing the prayer, he should perform sajdatā al‑sahw for standing without due reason. However, if he remembers [that he has not said tashahhud] during or after performing rukūʿ, then he must complete the prayer.
And based on recommended precaution, after the salām of the prayer, he should perform qaḍāʾ of the tashahhud, and he must perform sajdatā al‑sahw for the forgotten tashahhud.