The Quran

ثُمَّ أَنزَلَ اللَّهُ سَكينَتَهُ عَلىٰ رَسولِهِ وَعَلَى المُؤمِنينَ وَأَنزَلَ جُنودًا لَم تَرَوها وَعَذَّبَ الَّذينَ كَفَروا ۚ وَذٰلِكَ جَزاءُ الكافِرينَ

Muhammad Asad

whereupon God bestowed from on high His [gift of] inner peace upon His Apostle and upon the believers, and bestowed [upon you] from on high forces which you could not see,¹ and chastised those who were bent on denying the truth: for such is the recompense of all who deny the truth!

I.e., spiritual forces. Cf. 3 : 124-125 (relating to the battle of Uhud) and the corresponding note, as well as 8:9 (which refers to the battle of Badr). The spiritual nature of this aid is clearly implied in the phrase, "forces which you could not" [or "did not"] see".