It was narrated from Hafsah the wife of the Prophet (ﷺ) that:
the Messenger of Allah (ﷺ), said: "It is not permissible for a woman who believes in Allah and the Last Day to mourn for any deceased person for more than three days, except for her husband."
English reference: Vol. 3, Book 10, Hadith 2086
Arabic reference: Book 10, Hadith 2164