Musnad Ahmad 1153:158

Yahya narrated to us, Ibn Jurayj narrated to us, ‘Ata’ narrated to me, from ‘Ubayd ibn ‘Umayr, from ‘Aisha, who said: "The Messenger of Allah (peace and blessings of Allah be upon him) was not more diligent in observing any of the supererogatory prayers than the two rak'ahs before the morning prayer."