Safiyyah bint Abi Ubayd (may Allah be pleased with her) heard Hafsah bint Umar (may Allah be pleased with them both) narrating from the Prophet (peace and blessings be upon him) that the Prophet (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 days, except for her husband, for whom she mourns four months and ten days." Muslim narrated it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15521