Musnad Abu Ya'la al-Mawsili 108:171

Abu Khaythama narrated to us, Muhammad ibn Khazim narrated to us, from Hajjaj, from Muhammad ibn al-Munkadir, from Jabir ibn Abdullah, who said: A Bedouin came to the Prophet, may Allah bless him and grant him peace, and said: "Inform me about the Umrah; is it obligatory?" The Messenger of Allah, may Allah bless him and grant him peace, said: "No, and that you perform Umrah is better for you."