Apparently, Fatima Al-Zahra (peace be upon her) is the only daughter of the prophet (peace be upon him and his holy progeny). Others like Zainab and Rouqayyah are daughters of Khadijah. Some even say that they were Khadijaâ€™s step-daughters.
Intentionally reciting the third testimony during the tashahhud makes the prayer void, since it is not amongst the divinely-authorized words that are allowed to be recited in the prayer.
If he was unaware of the said ruling, his previous prayers are valid; also, they are valid if he recited it believing it to be recommended.