Musnad Ahmad 1:44

Muhammad ibn Maysar Abu Sa'd al-Saghani al-Makfuf narrated to us, Hisham ibn 'Urwah narrated to us, from his father, from 'Aishah, who said: When death approached Abu Bakr, he said: "What day is this?" They said: "Monday." He said: "If I die tonight, do not wait for the morrow for me, for the most beloved of days and nights to me is the one closest to the Messenger of Allah, may Allah bless him and grant him peace."