Musnad Ahmad 32:1868

Ahmad ibn Abd al-Malik narrated to us, Zuhayr narrated to us, Humayd al-Tawil narrated to us, from Anas, who said: There was a dispute between Khalid ibn al-Walid and Abd al-Rahman ibn Awf. Khalid said to Abd al-Rahman: "You boast against us because of days you preceded us in." It reached us that this was mentioned to the Prophet, peace and blessings of Allah be upon him, so he said: "Leave my companions for me. For by the One in Whose hand is my soul, if you were to spend the likes of Uhud—or the likes of the mountains—in gold, you would not reach their deeds."