Zaynab bint Abi Salamah narrates, through Umm Habibah (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 lawful for a woman who believes in Allah and the Last Day to mourn for a deceased person for more than three nights, except for a husband."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19727