Al-Zuhri said: Usamah bin Zayd (may Allah be pleased with him) continued to be addressed by (the title of) amir until he died. People used to say: The Messenger of Allah (peace and blessings be upon him) sent him as commander of an army, then he (peace and blessings be upon him) did not take that post away from him until he died.
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 368