Musannaf Ibn Abi Shayba 11:10

Ghundar narrated to us, from Shu'bah, from al-Hakam, who said: I heard 'Urwah ibn al-Nazzal narrating from Mu'adh ibn Jabal, who said: We arrived with the Messenger of Allah, may Allah bless him and grant him peace, from the expedition of Tabuk. I said: "O Messenger of Allah, tell me about its peak." He said: "As for its peak, it is Jihad in the way of Allah," meaning the peak of Islam.