Sahih Muslim Book 9, Hadith Number 3547.

Chapter: It is obligatory to abstain from adornment during the ‘Idda period, but mourning beyond three days is prohibited.

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.”