Safiyyah bint Abi Ubayd narrates, through Hafsah (may Allah be pleased with her), from the Prophet (peace and blessings be upon him) that he (peace and blessings be upon him) said: "It is not permissible for a woman who believes in Allah and His Messenger to mourn for more than three days, except for a husband."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19665