Sunan an-Nasa'i 3504
سنن النسائي#3504SahihIt was narrated from Safiyyah bint Abi 'Ubaid from one of the wives of the Prophet, and from Umm Salamah, that the Prophet said:"It is not permissible for a woman who believes in Allah and the Last Day to mourn for anyone who dies for more than three days except for a husband; she should mourn for him for four months and ten (days)
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Abu GhuddahSahih
Al-AlbaniSahih
Zubair Ali ZaiSahih
In-book reference: 27:116