Musnad Ahmad 1153:1588

Yahya narrated to us, Sa'id narrated to us, from Qatadah, from Mutarrif ibn 'Abdullah, from 'Aishah, that the Messenger of Allah (peace and blessings of Allah be upon him) used to say in his bowing and his prostration: "Subbuh, Quddus, Lord of the angels and the Spirit," three times. Then Yahya doubted regarding the "three times."