Musnad Ahmad 658:113

Hushaym narrated to us, Al-Hajjaj informed us, from Abu Ishaq, from Al-Bara’ ibn ‘Azib, who said: "It was among what the people of Makkah stipulated to the Messenger of Allah, may Allah bless him and grant him peace: that none of his companions shall enter it with a weapon, except for a weapon in a scabbard."