Musnad Ahmad 1157:4

'Abd al-Razzaq narrated to us, he said: Malik informed us, from 'Abdullah ibn Abi Bakr, from Humayd ibn Nafi', that Zaynab bint Abi Salamah informed him that she entered upon Zaynab bint Jahsh, the wife of the Prophet, may Allah bless him and grant him peace, and she said: "I heard the Messenger of Allah, may Allah bless him and grant him peace, upon the pulpit saying: 'It is not lawful for a woman who believes in Allah and the Last Day to mourn for a dead person for more than three nights, except for a husband, [for whom she mourns] four months and ten [days].'"