It is narrated from Hafsah (may Allah be pleased with her), or Aishah (may Allah be pleased with her), or both of them (may Allah be pleased with them both), that the Messenger of Allah (peace and blessings be upon him) said: "It is not permissible for a woman who believes in Allah and the Last Day, or who believes in Allah and His Messenger, to mourn for a deceased person for more than three days, except for her husband."
Muslim recorded it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15520