(16) باب بَيْنَ كُلِّ أَذَانَيْنِ صَلاَةٌ لِمَنْ شَاءَ
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).
Reference: Sahih al-Bukhari 627
In-book reference: Book 10, Hadith 24
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 1, Book 11, Hadith 600