Narrated 'Atiyyah: Abu Sa'eed narrated: "On the Day 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 - with the help of Allah... (30:1-5)'" He said: "So the believers were happy with the victory of the Romans over the Persians”"
Sahih LighairihiSahihHasanBook 47 · Hadith 244