Sahih al-Bukhari 627
Sahih al-Bukhari · Book 10: Call to Prayers (Adhaan) · Between every two calls (Adhan and Iqama) there is a Salat (prayer) (that is optional) for the one who wants to offer it.
Narrated `Abdullah bin Mughaffal:
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ يَزِيدَ ، قَالَ حَدَّثَنَا كَهْمَسُ بْنُ الْحَسَنِ ، عَنْ عَبْدِ اللَّهِ بْنِ بُرَيْدَةَ ، عَنْ عَبْدِ اللَّهِ بْنِ مُغَفَّلٍ ، قَالَ قَالَ النَّبِيُّ صلى الله عليه وسلم " بَيْنَ كُلِّ أَذَانَيْنِ صَلاَةٌ بَيْنَ كُلِّ أَذَانَيْنِ صَلاَةٌ ـ ثُمَّ قَالَ فِي الثَّالِثَةِ ـ لِمَنْ شَاءَ " .
The prophet said, "There is a prayer between the two Adhans (Adhan and Iqama), there is a prayer between the two Adhans." And then while saying it the third time he added, "For the one who wants to (pray).