Musnad Ahmad 1153:2155

Muhammad ibn Ubayd narrated to us, he said: Wa’il narrated to us, he said: I heard al-Bahiy narrating that Aisha said: "The Messenger of Allah, may Allah bless him and grant him peace, never sent Zayd ibn Harithah in an army at all, except that he appointed him over them. And if he had remained after him, he would have appointed him as his successor."