Answered by Sidi Fadi Qutub Zada
When does the late comer recite the opening supplication (thana)? For example if a latecomer joins the first rakaa’t before the imam goes into ruku, when should he pray this supplication? Should / can he pray it quickly before going into ruku or after standing up for the 2nd rakaa’t before the imam recites fatiha?
Answer:
In the Name of Allah, Most Merciful and Compassionate
Assalamu alaykum,
Ibn Abidin explained in his Radd al-Muhtar that:
If a latecomer joins a silent prayer while the imam is reciting in any of the raka’ts, then the opening supplication (thana) should be recited immediately.
Similarly, the opening supplication is also recited if one finds the imam in the first bowing (ruku) or prostration (sujud) of a quiet prayer, as long as one is able to catch him before he moves to the next position of the prayer.
In an audible prayer, however, one does not recite the opening supplication once the imam has begun reciting.
Fadi Qutub,
Amman, Jordan