Musnad Ahmad 1153:515

'Affan narrated to us, 'Abd al-Wahid ibn Ziyad narrated to us, he said: Al-Hasan ibn 'Ubayd Allah narrated to us, Ibrahim narrated to us, from Al-Aswad ibn Yazid, from 'Aishah, she said: "The Messenger of Allah, peace and blessings of Allah be upon him, used to exert himself during the ten [last days of Ramadan] in a way he did not exert himself at any other time."