once
يُصَلِّي عَلَى النَّبِيِّ صلى الله عليه وسلم بَعْدَ فَرَاغِهِ مِنْ إِجَابَةِ الْمُؤَذِّنِ
Meaning
One should then send prayers on the Prophet (sal-Allaahu 'alayhe wa sallam) after answering the call of the mu.adhdhin’.
Source and references
Hisn al-Muslim, entry 24.The reference is copied from the edition; separate editorial verification is not complete yet.
Source edition: Fortress of the Muslim. Open source ↗