Musnad Ahmad 1153:2102

'Abd al-Samad narrated to us, he said: Sulayman ibn Kathir narrated to us, from al-Zuhri, from 'Urwah, from 'Aishah, she said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "It is not permissible for a woman who believes in Allah and the Last Day to observe mourning for a dead person for more than three [days], except for a husband."