Musannaf Ibn Abi Shayba 27:481

Abu al-Jawwab narrated to us, he said: Yunus ibn Abi Ishaq narrated to us, from al-Bara’ ibn ‘Azib, who said: The Messenger of Allah (may Allah bless him and grant him peace) sent two armies, and over one of them was ‘Ali ibn Abi Talib, and over the other was Khalid ibn al-Walid. He said: "If there is fighting, then ‘Ali is over the people." ‘Ali conquered a fortress and took a slave girl for himself. Khalid wrote [a letter] complaining about him. When the Messenger of Allah (may Allah bless him and grant him peace) read the letter, he said: "What do you say about a man who loves Allah and His Messenger, and whom Allah and His Messenger love?"