Narrated Abu Sa'eed:
"On the Day of (the battle of) Badr, the Romans had a victory over the Persians. So the believers were pleased with that, then the following was revealed: Alif Lam Mim. The Romans have been defeated..." up to His saying: '...the believers will rejoice. (30:1-4)" He said: "So the believers were happy with the victory of the Romans over the Persians. (FR)
English reference (FR): Vol. 5, Book 43, Hadith 2935 (FR)
Arabic reference (FR): Book 46, Hadith 3186 (FR)