Reference : Hisn al-Muslim 219

قَالَ النَّبِيُّ صلى الله عليه وسلم: (مَنْ صَلَّى عَلَيَّ صَلاَةً صَلَّى اللَّهُ عَلَيْهِ بِهَا عَشْراً)
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The Prophet (ﷺ) said: "Whoever prays for Allah's blessings upon me once, will be blessed for it by Allah ten times."

Reference:
Muslim 1/288.

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