Musnad Abu Ya'la al-Mawsili 107:17

'Abd al-A'la ibn Hammad narrated to us, 'Abd al-'Aziz ibn Muhammad narrated to us, from Salih ibn Muhammad ibn Za'idah, from 'Umar ibn 'Abd al-'Aziz, from 'Uqbah ibn 'Amir, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "May Allah have mercy on the guard of the guards."