The Quran

إِذ يُريكَهُمُ اللَّهُ في مَنامِكَ قَليلًا ۖ وَلَو أَراكَهُم كَثيرًا لَفَشِلتُم وَلَتَنازَعتُم فِي الأَمرِ وَلٰكِنَّ اللَّهَ سَلَّمَ ۗ إِنَّهُ عَليمٌ بِذاتِ الصُّدورِ

Muhammad Asad

Lo! God showed them to thee in a dream as few:¹ for, had He shown them to you as many, you would surely have lost heart, and would surely have disagreed with one another about what to do.² But God saved [you from this]: verily, He has full knowledge of what is the hearts [of men].

Lit., "in thy dream" - obviously relating to a dream which the Prophet had had just before the encounter at Badr. We have no authentic Tradition to this effect, but the tabi'i Mujahid is quoted as having said, "God had shown the enemies to the Prophet, in a dream, as few; he informed his Companions accordingly, and this encouraged them greatly" (Razi and Ibn Kathir, with minor variants).
Lit., "about the matter" - i.e., about the advisability of giving battle or retreating.