"Aishah and Umm Salamah, the wives of the Prophet informed me that the Prophet would find that it was Fajr while he was Junub from (relations with) his wives, then he would perform Ghusl and fast." (Using translation from Tirmidhī 779)
عَنْ أُمِّ سَلَمَةَ وَعَائِشَةَ زَوْجِ النَّبِيِّ ﷺ أَنَّهُ كَانَ يُدْرِكُهُ الْفَجْرُ وَهُوَ جُنُبٌ مِنْ أَهْلِهِ ثُمَّ يَغْتَسِلُ وَيَصُومُ