ENOfficial
‘Abdallah b. ‘Amr said, “If anyone invokes blessing on the Prophet (ﷺ) once God and His angels will grant him seventy blessings.” Ahmad transmitted it.
Source: sunnah.com
Other translations
Arabic
وَعَنْ عَبْدِ اللَّهِ بْنِ عَمْرٍو قَالَ: مَنْ صَلَّى عَلَى النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ وَاحِدَةً صَلَّى اللَّهُ عَلَيْهِ وَمَلَائِكَتُهُ سَبْعِينَ صَلَاةً. رَوَاهُ أَحْمد
sunnah.com