I was doubtful on wether or not I have said attashhud at the first sitting after the second rakah and did prostration of forgetfulness before assalam (ending the prayer), but was still doubtful on whether that will suffice, so I stood up (still before assalam) and did one rakah to replace the mistake I think I made. Is my salah still valid?And what is the ruling on dozing off during salah and waking up almost immediately to continue salah.Thank you.

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

If you mean that you doubt whether you said the first Tashahud or forgot it, and you stood for the third Rak’ah and did not say the Tashahud, then your Prostration for Forgetfulness because of this doubt is legitimate according to some jurists, as is the Shafi’i Madh-hab. Mughni Al-Muhtaj reads: And if he had doubts about leaving some .. an essential part, he would prostrate because the basic rule is not to leave it. [End of Quote] However, adding one more Rak’ah is impermissible and it does not count as a makeup for the first Tashahud or even doubting it, and it is known that whoever adds a Rak’ah to his prayer deliberately, his prayer becomes invalidated, and according to some jurists it applies to anyone, whether he was aware of it or ignorant about it. While some other scholars hold the opinion that whoever added one more Rak’ah ignorantly, his prayer is not invalidated. According to this opinion, his prayer is valid, but still, to be on the safe side, it is better to repeat it.

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Allah knows best.