Sunan Ibn Majah
2086
Sahih+3

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

SahihAl-Albani
SahihMuhammad Fouad Abd al-Baqi