Reference : Hisn al-Muslim 220

وَقَالَ صلى الله عليه وسلم: (لاَ تَجْعَلُوا قَبْرِي عِيداً وَصَلُّوا عَلَيَّ؛ فَإِنَّ صَلاَتَكُم تَبْلُغُنِي حَيْثُ كُنْتُمْ)
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The Prophet (ﷺ) said: "Do not make my grave a place of ritual celebration, but pray for Allah's blessings upon me, for your blessings reach me from wherever you are.

Reference:
Abu Dawud 2/218, Ahmad 2/367. Al-Albani graded it authentic in Sahih Abu 
Dawud 2/383.

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