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."
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا نَصْرُ بْنُ عَلِيٍّ الْجَهْضَمِيُّ، حَدَّثَنَا الْمُعْتَمِرُ بْنُ سُلَيْمَانَ، عَنْ أَبِيهِ، عَنْ سُلَيْمَانَ الأَعْمَشِ، عَنْ عَطِيَّةَ، عَنْ أَبِي سَعِيدٍ، قَالَ لَمَّا كَانَ يَوْمُ بَدْرٍ ظَهَرَتِ الرُّومُ عَلَى فَارِسَ فَأَعْجَبَ ذَلِكَ الْمُؤْمِنِينَ فَنَزَلَتْ : ( الم *غَلَبَتِ الرُّومُ ) إِلَى قَوْلِهِ : (يفْرَحُ الْمُؤْمِنُونَ بِنَصْرِ اللَّهِ ) قَالَ فَفَرِحَ الْمُؤْمِنُونَ بِظُهُورِ الرُّومِ عَلَى فَارِسَ . قَالَ هَذَا حَدِيثٌ حَسَنٌ غَرِيبٌ مِنْ هَذَا الْوَجْهِ كَذَا قَرَأَ نَصْرُ بْنُ عَلِيٍّ : (غَلَبَتِ الرُّومُ ) .
sunnah.com