Musnad Ahmad 1153:1495

Hashim ibn al-Qasim narrated to us, he said: Warqa’ narrated to us, from ‘Abdullah ibn Dinar, he said: I heard Safiyyah saying: ‘Aishah, the wife of the Prophet, may Allah bless him and grant him peace, or Hafsah, or both of them, said: The Messenger of Allah, may Allah bless him and grant him peace, said: "It is not lawful for a woman who believes in Allah to mourn for more than three days, except for her husband."