Mishkat al-Masabih 4 Prayer كتاب الصلاة 94

وَعَنْ عَبْدِ اللَّهِ بْنِ مُغَفَّلٍ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «بَيْنَ كُلِّ أَذَانَيْنِ صَلَاةٌ بَيْنَ كُلِّ أَذَانَيْنِ صَلَاةٌ» ثُمَّ قَالَ فِي الثَّالِثَةِ «لِمَنْ شَاءَ»

‘Abdallah b. Mughaffal reported God’s Messenger as saying, “Between every pair of adhans there is a prayer; between every pair of adhans there is a prayer.” Then he said on repeating it a third time, “for him who wishes.” (Bukhari and Muslim.)