Musnad Ahmad 33:694

Khalaf ibn al-Walid narrated to us, 'Abbad ibn 'Abbad narrated to us, from Mujalid, from al-Sha'bi, from Jabir ibn 'Abd Allah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "You are today upon a religion, and I will boast of your numbers before the nations, so do not turn back on your heels after me."