Musnad Abu Ya'la al-Mawsili 149:68

Ishaq narrated to us, Sufyan narrated to us, from al-Zuhri, from Urwah, from Aisha, that the Prophet (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 mourn for a dead person for more than three [days], except for a husband."