There Is between the two calls (Adhan and Iqama) a prayer. And he mentioned it three times, and at the third time he said: This applies to those who wish to do it.  

قَالَ رَسُولُ اللَّهِ ﷺ بَيْنَ كُلِّ أَذَانَيْنِ صَلاَةٌ قَالَهَا ثَلاَثًا قَالَ فِي الثَّالِثَةِ لِمَنْ شَاءَ  

" He who wishes (may do to)."  

عَنِ النَّبِيِّ ﷺ مِثْلَهُ إِلاَّ أَنَّهُ قَالَ فِي الرَّابِعَةِ لِمَنْ شَاءَ