Musnad Ahmad 970:1

Khalaf ibn al-Walid narrated to us, Khalid narrated to us, from Sa'id al-Jurayri, from al-Sa'di, from his father, from his uncle, who said: "I observed the Messenger of Allah, peace and blessings of Allah be upon him, in his prayer. He would remain in his bowing (ruku') and his prostration (sujud) for the duration of what it takes to say: 'Subhan Allah wa bi-hamdihi' (Glory be to Allah and by His praise) three times."