Sahih Bukhari Volume 002, Book 023, Hadith Number 338.
Narated By Anas bin Malik : The Prophet said, "Zaid took over the flag and was martyred. Then it was taken by Jafar who was martyred as well. Then 'Abdullah bin Rawaha took the flag but he too was martyred and at that time the eyes of Allah's Apostle were full of tears. Then Khalid bin Al-Walid took the flag without being nominated as a chief (before hand) and was blessed with victory."