Zaynab bint Abi Salamah narrates from Umm Habibah (may Allah be pleased with her), who said: I heard the Messenger of Allah (peace and blessings be upon him) saying: "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, for whom she mourns four months and ten days."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19726