Musnad Ahmad 31:418

Muhammad ibn Ja'far narrated to us, Sa'id narrated to us, from Sulayman, from Abu al-Mutawakkil, from Abu Sa'id al-Khudri, that a man entered the mosque while the Messenger of Allah (peace and blessings of Allah be upon him) had already prayed with his companions. The Messenger of Allah (peace and blessings of Allah be upon him) said: "Who will give charity to this one and pray with him?" A man from the people stood up and prayed with him.