Musnad Ahmad 275:1

'Affan narrated to us, Khalid—meaning al-Wasiti—narrated to us, he said: 'Amr ibn Yahya al-Ansari narrated to us, from Ziyad ibn Abi Ziyad, the freed slave of Banu Makhzum, from a servant of the Prophet, peace and blessings of Allah be upon him, a man or a woman, who said: The Prophet, peace and blessings of Allah be upon him, used to say to the servant: "Do you have a need?" He said: Until one day he said: "O Messenger of Allah, my need." He said: "And what is your need?" He said: "My need is that you intercede for me on the Day of Resurrection." He said: "And who guided you to this?" He said: "My Lord." He said: "If that is the case, then help me by performing many prostrations."