Is this form of dhikr/duaa correct?

Salam, I’m not too sure what this would come under so I’ve assumed and put dhikr. It’s the duaa that you read after Maghrib and Fajr. ‘Allahumma Ajirnee Minannarr’ As a kid I was taught to read it 7 times but a post on Instagram said this is unreliable and not authentic Hadith. Can someone find out if this is true because he then stated 3 times is what Prophet muhammed PBUH read. Jazakallah Khair