Mishkat al-Masabih 7 Fasting كتاب الصوم 45

وَعَنْ عَائِشَةَ رَضِيَ اللَّهُ عَنْهَا قَالَتْ: كَانَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ يُدْرِكُهُ الْفَجْرُ فِي رَمَضَانَ وَهُوَ جُنُبٌ مِنْ غَيْرِ حُلْمٍ فَيَغْتَسِلُ وَيَصُومُ

She said that God’s messenger would be overtaken by the dawn in Ramadan when he was in a state of sexual defilement, not owing to a dream, and would wash and fast. (Bukhari and Muslim.)