Busr b. Mihjan reported his father as saying he had been at a meeting along with God’s Messenger when the adhan for prayer was called. God’s Messenger got up and prayed, and when he returned he found Mihjan sitting where he had been, so he asked him, “What prevented you from praying along with the people? Are you not a Muslim?” He replied, “Certainly, Messenger of God; but I had already prayed at home.” God’s Messenger then said to him, “When you come to the mosque having mready prayed and the time for prayer comes, pray along with the people even if you have already prayed.”
I was present with the Prophet at his pilgrimage and prayed the Morning Prayer along with him in the mosque of al-Khaif. When he finished his prayer and turned away there were two men at the back of the people who had not prayed along with him. He said, “Bring them to me,” and they were brought trembling with fear. He asked what had prevented them from praying along with him, and they replied, “Messenger of God, we had already prayed in our lodging.” He said, “Don’t do so. When you pray in your lodging and then come to a mosque where there is a congregation, you must pray along with them, and it will be a supererogatory prayer for you.
Narrated A man of Asad b. Khuzaima said he put this question to Abu Ayyub al-Ansari:
I* pray at home and then go to the mosque and the time to begin the prayer comes, so I pray along with the congregation, but I feel some uneasiness about that. Abu Ayyub replied, “We asked the Prophet about that and he said that such a person gets a portion of the reward for corporate prayer.”
* The Arabic has “One of us prays . . . then goes . . .” After that the first person singular is used. While such a construction is quite usual in 'Arabic, it makes impossible English, so I have used the first person all through.
He said that Mu’adh would pray the evening prayer along with the Prophet, then return to his people and lead them in the evening prayer, it being a supererogatory prayer for him…* transmitted it.
* The source is not mentioned. In al-Masabih, where the division of the chapters is into traditions which are sahih and traditions which are hasan, the collection of Tradition from which they are taken is not mentioned, but all those in section I are meant to be taken from Bukhari, or Muslim, or both. I have failed to find this one anywhere. The concordance does not seem to refer to it.