Reference : Hisn al-Muslim 223

وَقَالَ صلى الله عليه وسلم: (مَا مِنْ أَحَدٍ يُسَلِّمُ عَلَيَّ إِلاَّ رَدَّ اللَّهُ عَلَيَّ رُوحِيَ حَتَّى أَرُدَّ عَلَيْهِ السَّلاَمَ)
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The Prophet (ﷺ) said: "No one sends greetings (or prayers of peace) upon me but Allah returns my soul to me so that I may return his greetings."

Reference:
Abu Dawud (no. 2041). Al-Albani graded it good in Sahih Abu Dawud 1/383.

Chapter: Excellence of sending prayers upon the Prophet (saws)