Musnad Ahmad 32:882

Yunus ibn Muhammad narrated to us, Harb ibn Maymun narrated to us, from an-Nadr ibn Anas, from Anas, who said: I asked the Prophet of Allah, may Allah bless him and grant him peace, to intercede for me on the Day of Resurrection. He said: "I will do so." I said: "Where shall I seek you on the Day of Resurrection, O Prophet of Allah?" He said: "Seek me first at the Sirat (the bridge over Hell)." I said: "And if I do not meet you at the Sirat?" He said: "Then I am at the Mizan (the Scale)." I said: "And if I do not meet you at the Mizan?" He said: "Then I am at the Hawd (the Cistern). I will not miss these three places on the Day of Resurrection."