Musnad Abu Ya'la al-Mawsili 147:23

Abdullah ibn Muhammad ibn Asma’ narrated to us, Abdullah ibn al-Mubarak narrated to us, Sufyan narrated to us, from Zayd al-‘Ammi, from Abu Iyas, from Anas ibn Malik, that the Messenger of Allah (may Allah bless him and grant him peace) said: "For every nation there is a monasticism, and the monasticism of this nation is Jihad in the way of Allah."