Musnad Ahmad 1167:13

Mu'awiyah ibn 'Amr narrated to us, he said: Za'idah narrated to us, from Hisham ibn 'Urwah, from Fatimah bint al-Mundhir, from Asma', she said: "The Messenger of Allah, peace and blessings of Allah be upon him, commanded us to manumit slaves during the prayer of the solar eclipse."