Authentic
Sunan an-Nasa'i 1292
"I asked Zaid bin Kharijah who said: 'I asked the Messenger of Allah (ﷺ) and he said: Send salah upon me and strive hard in supplication, and say: Alahumma salli 'ala Muhammad wa 'ala ali Muhammad (O Allah, send salah upon Muhammad and upon the family of Muhammad).'"
" صَلُّوا عَلَىَّ وَاجْتَهِدُوا فِي الدُّعَاءِ وَقُولُوا اللَّهُمَّ صَلِّ عَلَى مُحَمَّدٍ وَعَلَى آلِ مُحَمَّدٍ "