Musnad Abu Ya'la al-Mawsili 209:64

Sufyan ibn Waki’ narrated to us, my father narrated to us, from Dawud, from Muhammad ibn Abd al-Rahman ibn Jud’an, from his grandmother, from Umm Salamah: That Abu al-Haytham al-Ansari came to the Messenger of Allah (ﷺ) and asked him for a servant. The Prophet (ﷺ) promised him that if he obtained captives, he would give him one. He met Umar and said to him, "O Abu al-Haytham, the Prophet (ﷺ) has obtained captives, so go to him and claim your promise." Abu al-Haytham and Umar went to the Messenger of Allah (ﷺ), and he said, "O Messenger of Allah, Abu al-Haytham has come to you to claim his promise." The Prophet (ﷺ) said to him, "We have obtained two black boys; choose whichever of them you wish." He said, "I seek your counsel." He replied, "The one consulted is trusted. Take this one, for he has prayed with us, and do not strike him, for we have been forbidden from striking those who pray."