Sunan An-Nasai, Chapter: The Book of Leading the Prayer (Al-Imamah)

that his father said: "I attended Fajr prayer with the Messenger of Allah (saw) in Masjid Al-Khaif.* When he finished praying, he saw two men at the back of the people who had not prayed with him. He said: 'Bring them here.' So they were brought to him, trembling. He said: 'What kept you from praying with us?' They said: 'O Messenger of Allah, we had already prayed in our lodgings.' He said: 'Do not do that. If you have already prayed in your lodgings, then you come to a Masjid in which there is a congregation, then pray with them, and it will be a voluntary prayer for you.'" (Sahih) *The main Masjid in Mina, one of stations of pilgrimage in Makkah.