If someone prays at home then joins the congregation, what should he do?

Hazrat Yazid bin Aswad Amiri says that I was present with the Messenger of Allah (peace and blessings be upon him) on the occasion of his Hajj. I performed the Fajr prayer with him in the Mosque of Khaif. When he completed the prayer and turned away from the Qiblah, there were two men at the back of the people who had not prayed with him. He said, "Bring both of them to me." When they were brought before him, they were trembling. He asked them, "Why did you not pray with us?" Both of them said, "O Messenger of Allah! We had already prayed in our tents." The Messenger of Allah (peace and blessings be upon him) said, "Do not do so. When you have prayed in your tents and then come to a mosque where the prayer is being held, then pray with them as well. That prayer will become a voluntary (nafl) prayer for you."

Hazrat Ibn Umar (may Allah be pleased with them both) narrates that only the prayer which he offered first will be counted.

Ibrahim says that the prayer he performed first will be considered obligatory, and this one will be considered voluntary.

Hazrat Sha'bi also says the same.

Hazrat Hakam bin A'raj says that I went to Hazrat Ibn Umar (may Allah be pleased with them). At that time, people were performing the Zuhr prayer. I thought that he did not have ablution. I said to him, "O Abu Abdur Rahman! Shall I bring water for you to perform ablution?" He said, "I have ablution and I have already performed the prayer. Which of the two should I count?" Yunus says that I mentioned this matter to Hasan, so he said, "May Allah have mercy on Abu Abdur Rahman! He considered the first prayer he performed as obligatory and the other as voluntary."

6811 - Yunus said: I mentioned that to Al-Hasan, and he said: May Allah have mercy on Abu Abd al-Rahman, he considers the first as the obligatory prayer and this as a voluntary (nafila) prayer.

Ibrahim says that when a person offers prayer alone and then offers prayer with the congregation, the obligatory prayer is the one which he performed first.