Musnad Ahmad 1060:93

Surayj narrated to us, Ibn Wahb narrated to us, from ‘Amr ibn al-Harith, from Bukayr, from Ibn Ta‘la, who said: We went on a military expedition with ‘Abd al-Rahman ibn Khalid ibn al-Walid. Four captives from the enemy were brought to him, and he ordered them to be killed by being tied up and shot with arrows. This reached Abu Ayyub, and he said: I heard the Messenger of Allah, may Allah bless him and grant him peace, forbidding the killing of captives tied up (al-sabir).