Musnad Ahmad 33:728

Mu'ammar ibn Sulayman al-Raqqi narrated to us, Hajjaj narrated to us, from Muhammad ibn al-Munkadir, from Jabir, who said: A man came to the Prophet, may Allah bless him and grant him peace, and said: "O Messenger of Allah, is the 'Umrah obligatory?" He said: "No."