Musnad Abu Ya'la al-Mawsili 210:4

Uthman ibn Abi Shaybah narrated to us, Abdah narrated to us, from Abdullah, from Nafi, from Safiyyah bint Abi Ubayd, from Hafsah bint Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "It is not lawful for a woman who believes in Allah and the Last Day to observe mourning for a dead person for more than three days, except for a husband."