When her brother died, she called for perfume and applied some of it. Then she said, "By Allah, I have no need for perfume, except that I heard the Messenger of Allah ﷺ say, 'It is not lawful for a woman who believes in Allah and the Last Day to mourn for any deceased person more than three nights, except for a husband, for whom she mourns four months and ten days.'"
قالت زينب فدخلت على
حين توفي أخوها فدعت بطيب فمست منه ثم قالت والله ما لي في الطيب من حاجة غير أني سمعت رسول الله ﷺ يقول لا يحل لامرأة تؤمن بالله واليوم الآخر أن تحد على ميت فوق ثلاث ليال إلا على زوج أربعة أشهر وعشرا
Agreed upon [by Bukhari and Muslim]. Bukhari narrated it in Talaq, chapter titled "The widow observes mourning for four months and ten days," hadith no. 5335, and Muslim narrated it, hadith no. 1487.