Musnad Abu Ya'la al-Mawsili 109:134

Ishaq narrated to us, Hammad narrated to us, Mujalid narrated to us, from Amir, from Jabir, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "He is not of us who raises his voice (in lamentation), shaves (the hair in grief), or tears (the garment)." And from Jabir, who said: The Messenger of Allah (may Allah bless him and grant him peace) 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."