وَقَالَ صلى الله عليه وسلم: ((مَا مِنْ أَحَدٍ يُسَلِّمُ عَلَيَّ إِلاَّ رَدَّ اللَّهُ عَلَيَّ رُوحِيَ حَتَّى أَرُدَّ عَلَيْهِ السَّلاَمَ)).
The Prophet [PBUH] 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
Abu Dawud (no. 2041). Al-Albani graded it good in Sahih Abu Dawud 1/383.