Sahih Muslim
The Book of Divorce (Kitab Al-Talaq)
Book Number: 009 / Hadith Number: 3547
Safiyya bint Abu 'Ubaid reported that she heard Hafsa daughter of Umar (Allah be pleased with them), (and) wife of Allah's Prophet (may peace be upon him), narrating a hadith like this from Allah's Apostle (may peace be upon him), and she made this addition:" She should abstain from adorning herself (in case of the death of her husband) for four months and ten days."