Musnad Abu Ya'la al-Mawsili 210:22

Abu Khaythama narrated to us, Jarir narrated to us, from Yahya ibn Sa'id, from Nafi', from Safiyya bint Abi 'Ubayd, from Hafsa bint 'Umar, 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 His Messenger, or who believes in Allah and the Last Day, to mourn for a dead person for more than three days, except for a husband."