"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."  

أَخْبَرَتْنِي عَائِشَةُ وَأُمُّ سَلَمَةَ زَوْجَا النَّبِيِّ ﷺ أَنَّ النَّبِيَّ ﷺ كَانَ يُدْرِكُهُ الْفَجْرُ وَهُوَ جُنُبٌ مِنْ أَهْلِهِ ثُمَّ يَغْتَسِلُ فَيَصُومُ