يُصَلِّي عَلَى النَّبِي صلى الله عليه وسلم بَعْدَ فَرَاغِهِ مِنْ إِجَابَةِ المُؤَذِّنِ.

One should send prayers on the Prophet ﷺ after answering the call of the muadhdhin.

Referencia: Muslim (#384, 1/288).