Musnad Ahmad 31:914

'Affan narrated to us, Hammam narrated to us, al-Mu'alla ibn Ziyad informed us, he said: Al-'Ala' - a man from Banu Muzaynah - narrated to me, from Abu al-Siddiq al-Naji, from Abu Sa'id al-Khudri, that they were sitting reciting the Qur'an and supplicating. He said: The Prophet, peace and blessings of Allah be upon him, came out to them. He said: When we saw him, we fell silent. He said: "Were you not doing such and such?" We said: "Yes." He said: "Then do as you were doing." He sat with us, then said: "Give glad tidings to the poor of the Emigrants (Muhajirun) of success on the Day of Resurrection over the wealthy by five hundred years." I believe he said: "years."