Sunan an-Nasā'ī 3540

The full narration

It was narrated from Zainab bint Abi Salamah, from Umm Salamah that a woman from the Quraish came to the Messenger of Allah and said:"My daughter's husband has died, and I am worried about her eyes; she needs kohl." He said: "One of you used to throw a piece of dung after a year had passed. Rather it (the mourning period) is four months and ten days." I (the narrator) said to Zainab: "What does 'after a year had passed' mean?" She said: "During the Jahiliyyah, if a woman's husband died she would go to the worst room she had and stay there, then, when a year had passed, she would come out and throw a piece of dung behind her