إِذْ تَسْتَغِيثُونَ رَبَّكُمْ فَاسْتَجَابَ لَكُمْ أَنِّي مُمِدُّكُم بِأَلْفٍ مِّنَ الْمَلَائِكَةِ مُرْدِفِينَ

Iz tastagheethoona Rabbakum fastajaaba lakum annee mumiddukum bi-alfim-minal-malaaa'ikati murdifeen.

[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." (Sura Anfal 8:9) He (Rasulullah ﷺ) continued his supplication to his Lord, stretching his hands, facing the Qibla, until his mantle slipped down from his shoulders. So Abu Bakr (RA) came to him, picked up his mantle and put it on his shoulders. Then he embraced him from behind and said: O Prophet of Allah (ﷺ), this prayer of yours to your Lord will suffice you, and He will fulfill for you what He has promised you. So Allah, the Glorious and Exalted, revealed (the Qur'anic verse): (The ayaat is mentioned above) So Allah helped him with angels.

Reference: Muslim: 1763