Musnad Ahmad 1155:16

Ya'qub narrated to us, my father narrated to us, from Ibn Ishaq, who said: It was narrated to me regarding the two rak'ahs after the dawn [break] before the morning prayer, from Nafi', from Ibn 'Umar, from Hafsa bint 'Umar, the wife of the Prophet, may Allah bless him and grant him peace, who said: "The Messenger of Allah, may Allah bless him and grant him peace, used to pray the two rak'ahs of the dawn before the morning prayer in my house, and he would make them very light." Nafi' said: And 'Abdullah used to make them light just the same.