حِصْنُ الْمُسْلِمِ
Hisn al-Muslim 26
Fortress of the Muslim (Hisn al-Muslim)
Concerning the athan (the call to prayer)
يَدْعُو لِنَفسِهِ بَيْنَ الْأَذَانِ وَالْإِقَامَةِ فَإِنَّ الدُّعَاءَ حِينَئِذٍ لاَ يُرَدُّ
Between the call to prayer and the 'Iqamah, you should supplicate Allah for yourself. Invocation during this time is not rejected. Reference: At-Tirmidhi, Abu Dawud, Ahmed. See Irwaa Al-Ghalil 1/262
Hisn al-Muslim, 26