Musnad Ahmad 1153:2274

'Abd al-Wahhab narrated to us, he said: Sa'id was asked, "What does a man say in his bowing (ruku')?" So he informed us from Qatadah, from Mutarrif ibn 'Abd Allah, from 'Aishah, that the Messenger of Allah, peace and blessings of Allah be upon him, used to say in his bowing and his prostration: "Subbuhun Quddusun, Rabbu al-Mala'ikati wa ar-Ruh" (Glorified, Holy, Lord of the angels and the Spirit).