Musannaf Ibn Abi Shaybah

Hazrat Yazid bin Aswad Amiri says that I was present with the Messenger of Allah (peace and blessings be upon him) on the occasion of his Hajj. I performed the Fajr prayer with him in the Mosque of Khaif. When he 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 both of them to me." When they were brought before him, they were trembling. He asked them, "Why did you not pray with us?" Both of them said, "O Messenger of Allah! We had already prayed in our tents." The Messenger of Allah (peace and blessings be upon him) said, "Do not do so. When you have prayed in your tents and then come to a mosque where the prayer is being held, then pray with them as well. That prayer will become a voluntary (nafl) prayer for you."