إِذْ تَسْتَغِيثُونَ رَبَّكُمْ فَاسْتَجَابَ لَكُمْ أَنِّي مُمِدُّكُم بِأَلْفٍ مِّنَ الْمَلَائِكَةِ مُرْدِفِينَ
“[Remember] when you asked help of your Lord, and He answered you, "Indeed, I will reinforce you with a thousand from the angels, following one another"”
The promise of 1,000 reinforcing angels at Badr.
This is the specific asbab al-nuzul: this ayah's promise of angelic reinforcement at Badr is what the post depicts -- the Prophet ﷺ pointing out Jibril leading the angels into the valley.