Musannaf Ibn Abi Shayba 36:331

Abu Bakr narrated to us, he said: Abd al-Rahim ibn Sulayman narrated to us, from Muhammad ibn Ishaq, from Ibn Umar, he said: The Messenger of Allah (may Allah bless him and grant him peace) sent us in a military expedition to Najd. He said: We obtained much livestock. He said: Our commander, who was over us, gave us as an extra reward (nafal) one camel each. Then we came to the Messenger of Allah (may Allah bless him and grant him peace) with what we had obtained. Our shares after the fifth were twelve camels each. So, for every man among us, there were thirteen camels, including the camel that our commander had given us as an extra reward. The Messenger of Allah (may Allah bless him and grant him peace) did not criticize our commander for what he had calculated for us regarding our shares.