Q: If I’m a bit late in salah, and the imam is about to go into ruku, which should I read: thanaa or fatiha? Is it okay to skip thanaa in this instance?
A: If the Imam is still in the Qiyaam position and reciting Qiraat, join the Salaah and do not recite anything; not Thanaa, nor Faatihah.
If he is already in the Ruku position, after joining in, join the Imam in Ruku. You will be allowed to read the Thanaa (after you join and while the Imam is in Ruku) if you are sure that after reciting Thanaa you will still get the Imam in Ruku). If you doubt, then do not read the Thanaa, but join the Imam in Ruku.
Surah Faatihah will not be recited.
Moulana Yusuf Laher