Sahih Muslim Book 9, Hadith Number 3540.

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

Zainab bint Umm Salama (Allah be pleased with her) reported that a relative of Umm Habiba (Allah he pleased with her) died. She sent for a yellow (perfume) and applied that to her forearm and said: I, am doing it, for I have heard Allah’s Messenger (may peace be upon him) saying: It is not permissible for a woman believing in Allah and the Hereafter to mourn beyond three days except the husband (for whom she can mourn) for four months and ten days, This hadith was narrated by Zainab from her mother and from Zainab, the wife of Allah’s Apostle (may peace be upon him), or from some other lady from among the wives of the Prophet (may peace be upon him).