What is obligatory for the ma'mum if he arrives while the imam is in ruku'

Q: If the ma'mum arrives at the prayer and the imam is in ruku', does he say the takbir of opening and the takbir of ruku', or does he say the takbir and go into ruku'?

A: The most preferable and safest course is to say the two takbirs:
The first is the takbir of ihram, which is a pillar, and he must perform it while standing.
The second is the takbir of ruku', which he says when he lowers himself into ruku'.
If he fears missing the rak'ah, the takbir of ihram suffices according to the more correct of the two opinions of the scholars; because they are two acts of worship that occurred at the same time, so the greater one suffices for the lesser, and this rak'ah is valid according to the majority of scholars; for al-Bukhari narrated in his Sahih from Abu Bakrah al-Thaqafi (may Allah be pleased with him) that he came to the Prophet (peace and blessings be upon him) while he was in ruku', so he went into ruku' before reaching the row, then he entered the row. The Prophet (peace and blessings be upon him) said to him: The meaning is: do not return to ruku' without being in the row; rather, the one entering should not go into ruku' until he reaches the row. He did not order him to make up the rak'ah, which indicates that it sufficed and that reciting al-Fatihah was dropped from his obligation; because its place, which is standing, was missed. This is the more correct opinion among those who said that reciting al-Fatihah is obligatory for the ma'mum.