One who prays at home and then finds a congregation

Sayyiduna Yazid bin al-Aswad al-Amiri (may Allah be pleased with him) said: I was present with the Messenger of Allah (peace and blessings be upon him) at his Hajj. I prayed the Fajr prayer with him in Masjid al-Khayf. When he had 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 the two of them to me." When they were brought before him, both of them were trembling. He asked them: "Why did you not pray with us?" They both said: "O Messenger of Allah, we had already prayed in our camps." He (peace and blessings be upon him) said: "Do not do that. When you have prayed in your camps and then come to a mosque where the prayer is being held, pray with them as well. That prayer will be voluntary for you."

Sayyiduna Ibn Umar (may Allah be pleased with them both) said: His prayer will be the one he prayed first.

Ibrahim said: The prayer he prayed first will be the obligatory one, and this one will be voluntary.

Al-Sha'bi also said the same.

Al-Hakam bin al-A'raj said: I came to Sayyiduna Ibn Umar (may Allah be pleased with them both) while the people were praying the Zuhr prayer. I thought he was not in a state of ablution, so I said to him: "O Abu Abd al-Rahman, shall I bring you water for ablution?" He said: "I am in a state of ablution, and I had already prayed; which of the two should I count?"

Yunus said: I mentioned this to al-Hasan, and he said: "May Allah have mercy on Abu Abd al-Rahman! He should count the first as the obligatory prayer and this one as voluntary."

Ibrahim said: When a man prays alone and then prays in congregation, the obligatory prayer is the one he performed first.