Musnad Ahmad 1153:1416

Bahz narrated to us, he said: Shu'bah narrated to us, from Qatadah, from Mutarrif, from 'Aishah, that the Messenger of Allah (may Allah bless him and grant him peace) used to say in his bowing (ruku'): "Subbuh, Quddus, Rabb al-Mala'ikah wa al-Ruh" (Exalted is He, Holy is He, Lord of the angels and the Spirit).