Musnad Ahmad 1153:262

Yahya narrated to us, Ibn Jurayj narrated to us, he said: 'Ata' narrated to me, from Ubayd ibn Umayr, from Aisha, she said: "The Messenger of Allah, peace and blessings of Allah be upon him, was not [as] strict in observing any of the supererogatory prayers as he was regarding the two rak'ahs before the morning [prayer]."