What is prescribed for one who enters while the imam is in prayer?

Question:
I see some of the worshippers, when they enter the mosque and the congregation is in the tashahhud, not begin entering the prayer until they are sure whether it is the first or the second tashahhud? If it is the first, they enter with the congregation, and if it is the second, they pray alone. I hope you will give a clear answer, and is it obligatory for one who enters the mosque and finds the people in prayer to enter with them in any case? And how many takbirs does he say if he finds them in ruku' or sujud? And does he recite the opening supplication before ruku' or sujud?

Answer:
The Sunnah for one who enters while the imam is in prayer is to pray so that he joins them, and to pray with them; because there are hadiths from the Messenger of Allah (peace and blessings be upon him) indicating that whoever comes while the imam is in a state, he does what the imam does. If he finds him standing, he lines up with him; if he finds him bowing, he bows; if he finds him prostrating, he prostrates; and if he finds him sitting, he sits. And he said regarding this—(peace and blessings be upon him): He says the takbir of ihram while standing, then he recites al-Fatihah if he is able, then he listens to the imam's recitation if it is loud, until... he bows and bows with him. And if it is silent, like Zuhr and 'Asr, he recites al-Fatihah with him, and recites what is easy for him in the first and second rak'ahs; because it is silent. And if he comes while he is bowing, he bows with him, saying takbir and bowing, as Abu Bakrah (may Allah be pleased with him) did . He came while the Prophet (peace and blessings be upon him) was bowing, so he bowed before the row, then entered the row. When the Prophet (peace and blessings be upon him) finished the prayer, he said to him: meaning: do not return to bowing before the row. This indicates that his rak'ah is sufficient, and that his prayer is valid, but it is not befitting for him to enter the prayer alone, rather he should join the row. If he is given time and it is possible for him to bow with them, he bows; otherwise, he makes up what he missed.
And what is prescribed is to say two takbirs: the first is the takbir of ihram, which is obligatory and necessary. The second is for ruku', if he intends to lower himself for ruku' and it is possible, he says the second takbir; because it is obligatory according to a group of scholars, and recommended according to the majority. It should be done if possible. If he fears missing the ruku', the first takbir, which is the takbir of ihram, is sufficient for him according to a group of scholars, and praise be to Allah; because when two acts of worship meet and one is greater than the other, the smaller enters into the greater, as in this case. Yes.

The presenter: May Allah reward you with good. Regarding the opening supplication?

Answer: The opening supplication is recommended. If he comes while the imam is standing and there is ample time, he opens with or with any type of the correct opening supplications, then he recites al-Fatihah. But if the time is tight and he fears that the imam will bow, he occupies himself with al-Fatihah and drops the opening supplication; because it is a Sunnah, and the obligatory is more important, al-Fatihah is more important, so he starts with it so that he does not miss the imam's bowing. Yes.

The presenter: May Allah reward you with good.