once
يَدْعُو لِنَفسِهِ بَيْنَ الْأَذَانِ وَالْإِقَامَةِ فَإِنَّ الدُّعَاءَ حِينَئِذٍ لاَ يُرَدُّ
Meaning
One should also supplicate for himself during the time between the athan and the iqamah as supplication at such time is not rejected.
Source and references
Hisn al-Muslim, entry 26.The reference is copied from the edition; separate editorial verification is not complete yet.
Source edition: Fortress of the Muslim. Open source ↗