From Sahih Bukhari (Hadith 5706):
Narrated Um Salama:The husband of a lady died and her eyes became sore and the people mentioned her story to the Prophet They asked him whether it was permissible for her to use kohl as her eyes were exposed to danger. He said, "Previously, when one of you was bereaved by a husband she would stay in her dirty clothes in a bad unhealthy house (for one year), and when a dog passed by, she would throw a globe of dung. No, (she should observe the prescribed period Idda) for four months and ten days
Reference: Bukhari 5706