Musnad Ahmad 1155:31

Isma'il narrated to us, Ayyub narrated to us, from Nafi', from Safiyyah bint Abi 'Ubayd, from some of the wives of the Prophet, may Allah bless him and grant him peace, who said: The Messenger of Allah, may Allah bless him and grant him peace, said: "It is not permissible for a woman who believes in Allah and the Last Day, or who believes in Allah and His Messenger, to observe mourning for a dead person for more than three [days], except for a husband, for she shall observe mourning for him for four months and ten [days]."