Yahya related to me from Malik from Nafi from Safiyya bint AbiUbayd from A'isha and Hafsa, the wives of the Prophet, may Allah blesshim and grant him peace, that the Messenger of Allah, may Allah blesshim and grant him peace, said, "It is not halal for a woman inmourning for someone who has died, if she trusts in Allah and the LastDay, to abstain from adornment for more than three nights, except fora husband."
USC-MSA web (English) reference: Book 29, Hadith 104
Arabic reference: Book 29, Hadith 1269