Musnad Ahmad 658:137

Yunus ibn Muhammad narrated to us, Fulayh narrated to us, from Safwan ibn Sulaym, from Abu Busrah, from Al-Bara’ ibn Azib, who said: "I went on a military expedition with the Messenger of Allah, may Allah bless him and grant him peace, for more than ten expeditions. I never saw him leave two rak'ahs when the sun inclined."