Musnad Ahmad 788:69

Yunus ibn Muhammad and ‘Affan narrated to us, they said: Hammad ibn Salamah narrated to us, from ‘Asim—‘Affan said: ‘Asim ibn Bahdalah informed us—from Abu Burdah, from Abu Musa: "That the Messenger of Allah, may Allah bless him and grant him peace, his companions used to guard him." And he mentioned the hadith.