It was narrated from 'Aishah that:
the Prophet (ﷺ) said: "It is not permissible for a woman to mourn for any deceased person for more than three days, except for her husband." (RU)
English reference (RU): Vol. 3, Book 10, Hadith 2085 (RU)
Arabic reference (RU): Book 10, Hadith 2163 (RU)