Musnad Ahmad 32:1863

Ya'mar narrated to us, Abdullah narrated to us, Sufyan informed us, from Zayd al-'Ammi, from Abu Iyas, from Anas ibn Malik, from the Prophet, peace and blessings of Allah be upon him, who said: "For every prophet there is a monasticism, and the monasticism of this nation is Jihad in the way of Allah."