Verse 56 of Surah Al-Ahzab, the basis for sending blessings (salawat) upon the Prophet.
Allah states that He and His angels send blessings upon the Prophet, and calls on the believers to do the same. This verse is why Muslims say 'peace and blessings be upon him' whenever the Prophet is mentioned, and it is the foundation of the durood recited in prayer.
- 33:56
إِنَّ ٱللَّهَ وَمَلَٰٓئِكَتَهُۥ يُصَلُّونَ عَلَى ٱلنَّبِىِّ ۚ يَٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوا۟ صَلُّوا۟ عَلَيْهِ وَسَلِّمُوا۟ تَسْلِيمًا
Innal laaha wa malaaa'i katahoo yusalloona 'alan Nabiyy; yaaa aiyuhal lazeena aamanoo salloo 'alaihi wa sallimoo tasleemaa
Indeed, Allah confers blessing upon the Prophet, and His angels [ask Him to do so]. O you who have believed, ask [Allah to confer] blessing upon him and ask [Allah to grant him] peace.