(55) باب عِدَّةِ الْمُتَوَفَّى عَنْهَا زَوْجُهَا
It 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)."
Reference: Sunan an-Nasa'i 3504
In-book reference: Book 27, Hadith 116
English translation: Vol. 4, Book 27, Hadith 3534