The ruling on one who catches the end of the congregation, then prays with another congregation

Question:
The questioner says: I came to the mosque for the Zuhr prayer, and I joined the prayer with the worshippers, but I was late, so I missed the last rak'ah. Then I stood up and completed the prayer, but...

The Shaykh: Repeat, repeat.

The presenter: He says: I came to the mosque for the Zuhr prayer, and I joined the prayer with the worshippers, but I was late, so I missed the last rak'ah. Then I stood up and made up the full prayer, but a few minutes after completing it, while I was reciting the dhikr, I noticed a new congregation in the mosque praying Zuhr. So I prayed with them, striving to do so, because they were a congregation, and because I had not caught up with the first congregation.
So my question here, O honorable Shaykh: Is my prayer valid in both cases? And which one is the one that suffices—if Allah wills—and which is better? Do I have to intend in my heart which one is the Zuhr prayer? May Allah reward you.

Answer:
Your prayer is valid, and the obligatory prayer is the first one that you made up after the imam. Your prayer with the second congregation is a voluntary prayer; because you were present for the prayer, you prayed with them as a voluntary prayer. The Prophet (peace and blessings be upon him) said: And do not say: "I have prayed, so I will not pray." If you pray with them, it is a voluntary prayer for you, and it is an abundance of good, and praise be to Allah. The obligatory prayer is the first one. Yes.