Musnad Ahmad 658:116

Hashim narrated to us, Layth narrated to us, Safwan ibn Sulaym narrated to us, from Abu Busrah, from Al-Bara’ ibn ‘Azib, who said: "I traveled with the Prophet, peace and blessings of Allah be upon him, eighteen journeys, and I did not see him abandon the two rak'ahs before the Dhuhr prayer."