Do not confuse for us the sunnah. Ibn al-Muthanna said, the sunnah of our Prophet ﷺ. The waiting period for a woman whose husband has died is four months and ten days, meaning the umm al-walad.
حدثنا قتيبة بن سعيد أن محمد بن جعفر حدثهم ح و حدثنا ابن المثنى حدثنا عبد الأعلى عن سعيد عن مطر عن رجاء بن حيوة عن قبيصة بن ذؤيب
لا تلبسوا علينا سنة قال ابن المثنى سنة نبينا ﷺ عدة المتوفى عنها أربعة أشهر وعشر يعني أم الولد
[Its isnad is daif]Ibn Majah narrated it in the book of divorce, chapter on the waiting period of a mother of a child (umm walad), no. 2083, from the hadith of Saeed bin Abu Aruba through this route. Ibn Hibban graded it sahih (Mawarid, 1333), as did Hakim, saying it meets the standard of the two Shaykhs (2/209), and Dhahabi agreed with him. Daraqutni said, "It is mursal, because Qabisa did not hear from Amr" (4/310).