بُلُوغُ الْمَرَامِ
Bulugh al-Maram 203
The Book of Prayer
وَعَنْ أَنَسٍ - رضى الله عنه - قَالَ : قَالَ رَسُولُ اَللَّهِ - صلى الله عليه وسلم -{ لَا يُرَدُّ اَلدُّعَاءُ بَيْنَ اَلْأَذَانِ وَالْإِقَامَةِ } رَوَاهُ النَّسَائِيُّ , وَصَحَّحَهُ اِبْنُ خُزَيْمَةَ . [1] .
Narrated Anas bin Malik (RA): Allah's Messenger (ﷺ) said: "A supplication made between the Adhan and Iqamah is not rejected." [Reported by an-Nasa'i and Ibn Khuzaimah graded it Sahih (authentic)].
Sunnah.com reference: Book 2, Hadith 66