Language:
كتاب الأذان
10  
Call to Prayers (Adhaan)
(16)
Chapter: Between every two calls (Adhan and Iqama) there is a Salat (prayer) (that is optional) for the one who wants to offer it.
(16)
باب بَيْنَ كُلِّ أَذَانَيْنِ صَلاَةٌ لِمَنْ شَاءَ
Sahih al-Bukhari 627

Narrated `Abdullah bin Mughaffal:

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).

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 ‏"‏‏.‏
Reference : Sahih al-Bukhari 627
In-book reference : Book 10, Hadith 24
USC-MSA web (English) reference : Vol. 1, Book 11, Hadith 600
  (deprecated numbering scheme)