Sahih Muslim 3735
صحيح مسلم#3735SahihSafiyya bint Abu 'Ubaid reported on the authority of Hafsa or 'A'isha (Allah be pleased with thein) or from both of them that Allah's Messenger (may peace he upon him) said:It is not permissible for a woman believing in Allah and the Hereafter (or believing in Allah and His Messenger) that she should observe mourning for the dead beyond three days except in case of her husband
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Every narration in Sahih Muslim is accepted as Sahih (authentic) by the established status of the book.
In-book reference: 18:80