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Hisn al-Muslim (Fortress of the Muslim) 24
((يُصَلِّي عَلَى النَّبِيِّ صلى الله عليه وسلم بَعْدَ فَرَاغِهِ مِنْ إِجَابَةِ الْمُؤَذِّنِ)).
‘One should then send prayers on the Prophet (sal-Allaahu 'alayhe wa sallam) after answering the call of the mu.adhdhin’.
Cited in Hisn al-Muslim (Fortress of the Muslim) (No. 24).