This hadith was narrated by Zainab from her mother and from Zainab, the wife of Allah's Apostle (ﷺ), or from some other lady from among the wives of the Prophet (ﷺ).
Sahih Muslim Book 18, Hadith 75
← Previous Hadith I, am doing it, for I have heard Allah's Messenger (ﷺ) 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.

Sahih Muslim Book 18, Hadith 74 Next Hadith → One among you used to spend one year in a dungeon dressed in worst clothes. (And at the end of this period) she threw dung at the dog which happened to pass that way and then she came out (of her 'Idda). Can't she (wait) even for four months and ten days?

Sahih Muslim Book 18, Hadith 76