Musnad Ahmad 33:555

Hassan narrated to us, Ibn Lahi'ah narrated to us, Abu al-Zubayr narrated to us, he said: I asked Jabir, "Did you hear the Messenger of Allah, may Allah bless him and grant him peace, say: 'Do not fast until you see the crescent, and if it is hidden from you, then complete thirty [days]'?" And Jabir said: The Messenger of Allah, may Allah bless him and grant him peace, stayed away from his wives for a month, then he came down after twenty-nine [days], and he said: "The month is indeed twenty-nine [days]."