Musnad Ahmad 30:684

Ibrahim ibn Khalid narrated to us, Rabah narrated to us, from Ma'mar, from al-Zuhri, Abu Salamah ibn 'Abd al-Rahman informed me, that Abu Hurayrah said: The Messenger of Allah (peace and blessings of Allah be upon him) stood for prayer, and we stood with him. A Bedouin said while he was in the prayer: "O Allah, have mercy on me and on Muhammad, and do not have mercy on anyone else with us." When the Prophet (peace and blessings of Allah be upon him) finished the prayer, he said to the Bedouin: "You have restricted something vast," meaning the mercy of Allah.