It is narrated from Sayyidah Umm Salamah (may Allah be pleased with her) that the Prophet (peace and blessings be upon him) said: "It is not permissible for a woman to mourn for more than three days, except for a woman (mourning) her husband for four months and ten days."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20313