Reference : Hisn al-Muslim 24

يُصَلِّي عَلَى النَّبِيِّ صلى الله عليه وسلم بَعْدَ فَرَاغِهِ مِنْ إِجَابَةِ الْمُؤَذِّنِ
After replying to the call of Mu'adh-dhin. you should recite in Arabic Allah's blessings on the Prophet (ﷺ).

Reference:
Muslim 1/288

Chapter: Concerning the athan (the call to prayer)