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.
The fact that you uttered ‘Allahu Akbar (Takbir)’ with the intention of sitting, does not affect the validity of your prayer. This Takbir serves only as a Takbirat Intiqal (Takbir of transitioning from one position to another). Should you realize that you are still in the first Rak’ah and that the Tashahhud is not yet due, you should stand up for the second Rak’ah without pronouncing another Takbirat Intiqal, as the Takbirat Intiqal has already been made. If you inadvertently sat down briefly, even for a moment equivalent to a restful sitting, then you are required to perform the Prostration of Forgetfulness at the conclusion of your prayer, according to the scholars of the Hanbali School of Fiqh (jurisprudence). However, if you did not sit at all, no further action is required.
Allah knows best.