the Prophet said: "Between every two calls (to prayer) there is a Salat for whoever wills."  

عَنِ النَّبِيِّ ﷺ قَالَ بَيْنَ كُلِّ أَذَانَيْنِ صَلاَةٌ لِمَنْ شَاءَ وَفِي الْبَابِ عَنْ عَبْدِ اللَّهِ بْنِ الزُّبَيْرِ