My father and I visited Ayesha and Umm Salama, and they said, "The Prophet ﷺ would wake up in a state of major ritual impurity and then fast."
حدثنا عبد الأعلى عن معمر عن الزهري
دخلت أنا وأبي على عائشة وأم سلمة فقالتا إن النبي ﷺ كان يصبح جنبا ثم يصوم
Its isnad is sahih. It is found in Bukhari 4/143 no. 1925, Muslim 2/781 no. 1109, and Tirmidhi 2/140 no. 779, where he said, hasan sahih. Also Abu Dawud 2/312 no. 2388 and Ibn Majah 1/543 no. 1703, all of them in the chapter on fasting.