Muhaqqaq Masala

If a latecomer (Masbooq) joins the Imam in the prayer during the first sitting (Qa'dah Ula), and as soon as he sits in the Qa'dah, the Imam stands up for the third Rak'ah, then the Masbooq should stand up after reciting Attahiyat. This is because Tashahhud has become obligatory (Wajib) upon the Masbooq by following the Imam, and standing up without reciting Attahiyat is Makrooh Tahrimi (severely disliked). However, if someone stands up (without reciting it), the prayer will still be valid. (Aham Masail / Vol: 1 / Page: 44)