Al-Kāfi
Hadith 494عَنْهُ عَنْ بَعْضِ أَصْحَابِنَا فِي الْمُطَلَّقَةِ الْبَائِنَةِ إِذَا تُوُفِّيَ عَنْهَا وَ هِيَ فِي عِدَّتِهَا قَالَ تَعْتَدُّ بِأَبْعَدِ الْأَجَلَيْنِ .
2. It is narrated from the narrator of the previous Hadith from certain persons of our people who has said the following: “About the case of a woman divorced irrevocably if her husband dies, he (the Imam), ‘Alayhim al- Salam, has said, ‘She must count her waiting period according to the longer of the two forms of waiting periods.’”