Answered by Shaykh Faraz Rabbani
If a Hanafi is praying asr and dose not sit down intentionally in the 2nd rak’a and read attahiyat then is his salat valid? if he forgets to sit down and prays on and remembers after salat that he didn’t sit down after 2nd rak’a is it then valid?
Prayer: Omitting The First Sitting
The first sitting is necessary (wajib). It is also necessary to recite the tashahhud in it. [Maraqi]
As a result, omitting this does not invalidate the prayer. If it was done forgetfully, one must perform forgetfulness prostrations at the end of the prayer. If done intentionally, one must repent and repeat the prayer. [Maraqi, Radd al-Muhtar]