What is the ruling on one who delays reciting the first tashahhud, then bows with the Imam who bows before he completes the tashahhud, justifying that by mixing up the fatwa regarding the obligation of completing the first tashahhud and the fatwa regarding bowing with the Imam?

The congregant is not permitted to delay behind their Imam without an excuse, and must follow him. Scholars have differed regarding the obligation of reciting Surat Al-Fatiha and the Tashahhud for the congregant. Those who consider it a Sunnah (recommended act) do not permit the congregant to fall behind the Imam. However, those who consider them obligatory must complete them and then catch up with their Imam. The obligatory portion of the Tashahhud, according to those who deem it obligatory, consists of only five words.