Sayyiduna Anas (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) said: "It is not lawful for any woman who believes in Allah Almighty and the Last Day to mourn for anyone's death for more than three days, except in the case of a husband."
Al-Bazzar recorded it. Its chain contains a narrator, Zam'ah bin Salih, who is da'if (weak), although he has been declared trustworthy.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الطلاق / 7812