Musnad Abu Ya'la al-Mawsili 152:277

And through it, Abu 'Awanah narrated to us, from 'Abd al-Malik ibn 'Umayr, from Khalid ibn Rib'i al-Asadi, who said: I heard Ibn Mas'ud saying: I heard the Messenger of Allah (may Allah bless him and grant him peace) saying: "Indeed, your companion is the Khalil (intimate friend) of Allah."