Musnad Ahmad 1153:1346

'Abd al-Razzaq and Ibn Bakr narrated to us, they said: Ibn Jurayj informed us, from 'Ata', that he was informed by 'Ubayd ibn 'Umayr, from 'A'ishah, that she said: "The Prophet, peace and blessings of Allah be upon him, was not more diligent regarding any of the supererogatory prayers than he was regarding the two rak'ahs before the morning prayer." I heard this from 'Ata' repeatedly.