"It is not permissible for a woman who believes in Allah and the Last Day to mourn for anyone who dies for more than three days except for a husband; she should mourn for him for four months and ten (days)."
Sunan an-Nasa'i Book 27, Hadith 115
← Previous Hadith "A woman came to the Prophet and said: 'My daughter's husband has died, and I am worried about her eyes. Can I apply kohl to her?' The Messenger of Allah said: 'One of you used to stay (in mourning) for a year. Rather (the mourning period is) four months and ten (days). And when that year had passed she would go out and fling a piece of dung behind her.'"

Sunan an-Nasa'i Book 27, Hadith 114 Next Hadith → "It is not permissible for a woman who believes in Allah and the Last Day to mourn for anyone who dies for more than three days except for a husband; she should mourn for him for four months and ten (days)."

Sunan an-Nasa'i Book 27, Hadith 116