When the call for Subh prayer was given, the Messenger of Allah ﷺ would pray two brief rak'ahs before going to the prayer.  

عَنْ رَسُولِ اللَّهِ ﷺ أَنَّهُ كَانَ إِذَا نُودِيَ لِصَلاَةِ الصُّبْحِ رَكَعَ رَكْعَتَيْنِ خَفِيفَتَيْنِ قَبْلَ أَنْ يَقُومَ إِلَى الصَّلاَةِ  

"Hafsah told me that when dawn glowed, the Prophet ﷺ would pray two rak'ahs.  

أَنَّ النَّبِيَّ ﷺ كَانَ إِذَا أَضَاءَ لَهُ الْفَجْرُ صَلَّى رَكْعَتَيْنِ