Assalamu alaikum,
My question is what is the ruling on a person praying, and during the first rakaa that is after reciting fatiha, he remenber he did not say Takbirat.

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

Takbeeratul-Ihraam is one of the obligations of the prayer without which the prayer is void. Therefore, whoever forgets to say the Takbeeratul-Ihraam and starts reciting Al-Faatihah, and then he remembers, he is obliged to say Takbeeratul-Ihraam as he has not started the prayer in principle and the Prophet said: "Entering the prayer is by saying Allaahu Akbar and finishing the prayer is by saying Tasleem (As-Salaamu ‘Alaykum wa Rahmatullaah).” [Abu Daawood, Ahmad and At-Tirmithi]

An-Nawawi said: “The prayer is void without Takbeeratul-Ihraam, and if a person who leads the prayer or the one who is led in the prayer abandons it out of forgetfulness or deliberately, then his prayer does not materialize and the Takbeer for Ruku’ (bowing down) or other Takbeers are not sufficient instead of it. This is our view and this is the view of Abu Haneefah, Maalik, Ahmad, Daawood and the view of the majority of the scholars…”

Therefore, if someone forgets Takbeeratul-Ihraam and starts reciting Al-Faatihah, he is obliged to interrupt his prayer and then say Takbeeratul-Ihraam and then recite again Al-Faatihah and continue his prayer.

Allaah Knows best.