Muslims invoke Allah for Help, Allah sends the Angels to help Them

Al-Bukhari wrote in the book of battles (in his Sahih) under "Chapter; Allah's statement,

إِذْ تَسْتَغِيثُونَ رَبَّكُمْ فَاسْتَجَابَ لَكُمْ

The fourth verse (10) restates this aspect explicitly by saying: وَمَا جَعَلَهُ اللَّـهُ إِلَّا بُشْرَ‌ىٰ وَلِتَطْمَئِنَّ بِهِ قُلُوبُكُمْ. It means: 'Allah has done it only to give you glad tidings and so that your hearts might be at rest thereby.'

The number of angels sent to support Muslims in the battle of Badr has been given as one thousand at this place, while in Surah 'Al-` Imran (3:124) the number mentioned is three thousand and five thousand. The reason for this lies in three different promises made under different circumstances. The first promise was that of one thousand angels, the reason for which was the prayer of the Holy Prophet ﷺ and the supporting plaint of common Muslims with him. The second promise of three thousand angels which appears first in Surah 'Al-` Imran was made at a time when Muslims heard about the additional forces coming to join the Quraysh army. It has been reported in Ruh al-Ma'ani as based on a narration of Al-Sha'bi from Ibn Abi Shaybah, Ibn al-Mundhir and others that on the day of the battle of Badr when Muslims heard that Kurz ibn Jabir Muharibi was coming with additional armed support for the disbelievers, they were disturbed and agitated. Thereupon, the verse of 'Al-` Imran: أَلَن يَكْفِيَكُمْ أَن يُمِدَّكُمْ رَ‌بُّكُم بِثَلَاثَةِ آلَافٍ مِّنَ الْمَلَائِكَةِ مُنزَلِينَ : "Shall it not suffice you that your Lord should help you with three thousand of the angels sent down (for you)?" - 3:124) was revealed and referred to wherein is the promise of sending a force of three thousand angels from the heavens in order to support believers.