Silsilat al-Ahadith al-Sahihah

Busr bin Mihjan narrates from his father Sayyiduna Mihjan (may Allah be pleased with him) that he was present in a gathering with the Messenger of Allah (peace and blessings be upon him). The call to prayer was given, so the Prophet (peace and blessings be upon him) stood up and performed the prayer. When he returned (after praying), he saw that Mihjan was still sitting there; he had not prayed with the Prophet (peace and blessings be upon him). The Prophet (peace and blessings be upon him) asked him: "What prevented you from praying with the people? Are you not a Muslim?" He said: "O Messenger of Allah! Why not, (I am a Muslim, in fact) I had already performed the prayer at home." The Prophet (peace and blessings be upon him) said: "When you come (and the people are praying), then pray with them, even if you have already performed the prayer."