Musnad Abu Ya'la al-Mawsili 210:2

Abdullah narrated to us, Juwayriyah narrated to us, from Nafi’, that Safiyyah bint Abi Ubayd narrated to him, from Hafsah or from ‘Aishah, or from both of them, that the Messenger of Allah (may Allah bless him and grant him peace) said: "It is not lawful for a woman who believes in Allah and the Last Day, or who believes in Allah and His Messenger, to mourn for a dead person for more than three days, except for her husband."