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.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا نَصْرُ بْنُ عَلِيٍّ الْجَهْضَمِيُّ، قَالَ 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 * غُلِبَتِ الرُّومُ ) إِلَى قَوْلِهِ (يَفْرَحُ الْمُؤْمِنُونَ ) قَالَ فَفْرَحَ الْمُؤْمِنُونَ بِظُهُورِ الرُّومِ عَلَى فَارِسَ . قَالَ أَبُو عِيسَى هَذَا حَدِيثٌ حَسَنٌ غَرِيبٌ مِنْ هَذَا الْوَجْهِ . وَيُقْرَأُ غَلَبَتْ وَغُلِبَتْ يَقُولُ كَانَتْ غُلِبَتْ ثُمَّ غَلَبَتْ هَكَذَا قَرَأَ نَصْرُ بْنُ عَلِيٍّ غَلَبَتْ .
sunnah.com