Musannaf Ibn Abi Shayba 33:808

Muhammad ibn Bishr and Abu Usamah narrated to us, from Mis'ar, from 'Amr ibn Murrah, from Mus'ab ibn Sa'd. And Abu Usamah said: I heard Mus'ab ibn Sa'd saying: "When the servant says 'Subhan Allah' (Glory be to Allah), the angels say 'wa bihamdihi' (and with His praise). And when he says 'Subhan Allah wa bihamdihi' (Glory be to Allah and with His praise), they pray for him." And Abu Usamah said: "They pray for his forgiveness."