Is the prayer valid for a follower who completed the first Tashahhud after the Imam had risen for the third Rak'ah, then caught up with the Imam in the bowing (Ruku') without completing the Fatiha, knowing that completing the Tashahhud (which is obligatory) prevented him from completing the Fatiha (which is a pillar)?

If the Imam stands up for the third rak'ah and the follower has not completed the first tashahhud, then according to some scholars, the follower completes it and then joins the Imam. However, if the follower has not completed Al-Fatiha before the Imam bows, the Shafi'is hold that he completes Al-Fatiha. Whereas many scholars, including Ibn ‘Uthaymeen, are of the view that the follower bows in 따라 his Imam and does not lag behind to complete Al-Fatiha, and his prayer is valid. Ibn ‘Uthaymeen believes that the follower should not lag behind his Imam even to complete the first tashahhud, because following the Imam takes precedence.