Musnad Ahmad 1153:2169

Abu Sa’id narrated to us, he said: ‘Abd al-Wahid ibn Ziyad narrated to us, from al-Hasan ibn ‘Ubayd Allah, from Ibrahim, from al-Aswad, from ‘A’ishah, who said: "The Messenger of Allah (may Allah bless him and grant him peace) used to exert himself in the last ten [nights of Ramadan] in a way that he did not exert himself in any other [time]."