The Messenger of Allah ﷺ addressed the people and said, "Zayd took the banner and was struck down, then Jafar took it and was struck down, then Abdullah bin Rawaha took it and was struck down," while his eyes were overflowing with tears. Then he said, "Then Khalid took it without being appointed, and Allah granted victory through him. It would not please me that they were here with us," or he said, "It would not please them that they were with us."
حدثنا إسماعيل حدثنا أيوب عن حميد بن هلال
خطب رسول الله ﷺ فقال أخذ الراية زيد فأصيب ثم أخذها جعفر فأصيب ثم أخذها عبد الله بن رواحة فأصيب وإن عينيه لتذرفان ثم أخذها خالد من غير إمرة ففتح الله عليه وما يسرني أنهم عندنا أو قال ما يسرهم أنهم عندنا
Its chain of transmission is sahih. Ismail is Ibn Ulaya mentioned earlier, Ayub is al-Sakhtiyani, and Humayd bin Hilal is the judge and jurist. Bukhari related the hadith in 4/21, in the chapter on jihad, wishing for martyrdom, and Bayhaqi related it in Dalail al-Nubuwwah 4/367.