Fortress of the Muslim
Hisn al-Muslim 26
Arabic hadith text
يَدْعُو لِنَفسِهِ بَيْنَ الْأَذَانِ وَالْإِقَامَةِ فَإِنَّ الدُّعَاءَ حِينَئِذٍ لاَ يُرَدُّ
Translation
He should supplicate for himself between the call to prayer and the commencement of the prayer, for supplication at that time is not rejected.