From Sahih Bukhari (Hadith 5338):
Narrated Um Salama: A woman was bereaved of her husband and her relatives worried about her eyes (which were diseased). They came to Allah's Messenger (ﷺ), and asked him to allow them to treat her eyes with kohl, but he said, "She should not apply kohl to her eyes. (In the Pre-Islamic period of Ignorance) a widowed woman among you would stay in the worst of her clothes (or the worst part of her house) and when a year had elapsed, if a dog passed by her, she would throw a globe of dung, Nay, (she cannot use kohl) till four months and ten days have elapsed
Reference: Bukhari 5338