(9) باب وُجُوبِ الإِحْدَادِ فِي عِدَّةِ الْوَفَاةِ وَتَحْرِيمِهِ فِي غَيْرِ ذَلِكَ إِلاَّ ثَلاَثَةَ أَيَّامٍ
Safiyya 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.
Reference: Sahih Muslim 1490 a
In-book reference: Book 18, Hadith 80
USC-MSA web (English) reference(deprecated numbering scheme): Book 9, Hadith 3545