He offered the morning prayer with Allah's Messenger (ﷺ) and when Allah's Messenger (ﷺ) finished his prayer, he saw two men who had not prayed with him. He ordered them to be brought and they were brought trembling with fear. He asked them what had prevented you from praying with us?" They said, "We had already prayed at our homes." He said, "Don't do so! If you pray at your homes and then you come while the Imam has not yet performed the prayer, you must pray with him, and it will be a voluntary prayer for you." [Reported by Ahmad - with his wording - and ath-Thalathah. at-Tirmidhi and Ibn Hibban graded it Sahih (authentic)].