When late joining a Jamaa’ah (congregational) prayer in Rukoo’ position, can we consider that as one Rak'ah or continue after Imam's Salaam for another one Rak'ah?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

There is a difference of opinion among the scholars about reciting al-Faatihah by a follower whether or not it is a pillar of the prayer. This, in turn, leads to another difference of opinion concerning when joining the Imam in the Rakoo’ (bowing down position) whether or not it constitutes one Rak'ah.

Anyway, according to the predominant opinion of the majority of the Muslim scholars, whoever reaches the Rukoo’ with the Imam even for a moment of calm, then he is considered as having reached the Rak'ah.

In this vein, the Prophet said: “If you come to the prayer and we are in prostration, then make prostration with us but do not count it (as a Rak'ah). And whoever 'catches' the Rukoo’, he catches the prayer.” [Abu Daawood from Abu Hurayrah, and Ibn Khuzaymah classified it as sound]

This Hadeeth means that whoever performs Rukoo’ with the Imam, has reached the Rak'ah. This is also confirmed in another narration of the above-Hadeeth reported by Imam Ahmad, which reads: "Whoever catches the Rukoo’, he catches the Rak'ah."

Abu Bakrah narrated that he came to the prayer while the Prophet was performing Rukoo’, and he (Abu Bakrah) performed the Rukoo’ before he entered the row. Then, he mentioned this to the Prophet who said: “May Allah increase your love for goodness, but do not repeat that act again.” [Al-Bukhaari]

This Hadeeth indicates that one who catches the Rukoo’, catches the Rak'ah, since the Prophet did not order Abu Bakrah to perform the Rak'ah again because it is sufficient.

Allah knows best.