"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)
أَنَّ أُمَّ سَلَمَةَ وَعَائِشَةَ زَوْجَتَيِ النَّبِيِّ ﷺ أَخْبَرَتَاهُ أَنَّ رَسُولَ اللهِ ﷺ «كَانَ يُدْرِكُهُ الْفَجْرُ وَهُوَ جُنُبٌ مِنْ أَهْلِهِ ثُمَّ يَغْتَسِلُ فَيَصُومُ»