Abdullah bin Abbas reported,

The verse was recited both as "were defeated" and as "were victorious." The polytheists wanted the Persians to prevail over the Romans, since they and the Persians were both idolaters, while the Muslims wanted the Romans to prevail over the Persians, since the Romans were People of the Book. The matter was mentioned to Abu Bakr, and Abu Bakr mentioned it to the Messenger of Allah ﷺ, who said, "They will certainly be victorious."Abu Bakr repeated this to the polytheists, and they said, "Then set a term between us and you. If we win, we get such and such, and if you win, you get such and such."He set a term of five years, but the Romans did not prevail within it. This was mentioned to the Prophet ﷺ, who said, "Why did you not set it at something less?" The narrator said, I think he meant less than ten. Abu Saeed said that "bid" means anything under ten. He said, Then the Romans did prevail afterwards.That is the meaning of Allah's words, "Alif Lam Mim. The Romans have been defeated" [Rum 30:1–2], down to His words, "the believers will rejoice in the victory of Allah. He gives victory to whom He wills" [Rum 30:4–5].Sufyan said, I heard that they prevailed over them on the day of Badr.

في قول الله تعالي{الم غلبت الروم في أدن الأرض}

عن ابن عباس قال

غلبت وغلبت، كان المشركون يحبون أن يظهر أهل فارس على الروم، لأنهم وإياهم أهل أوثان، وكان المسلمون يحبون أن يظهر الروم على فارس، لأنهم أهل كتاب، فذكروه لأبي بكر، فذكره أبو بكر لرسول الله ﷺ قال: أما إنهم سيغلبون فذكره أبو بكر لهم فقالوا: اجعل بيننا وبينك أجلا، فإن ظهرنا كان لنا كذا وكذا، وإن ظهرتم كان لكم كذا وكذا، فجعل أجل خمس سنين فلم يظهروا، فذكر ذلك للنبي ﷺ قال: ألا جعلته إلى دون. قال: أراه العشر، قال أبو سعيد: والبضع ما دون العشر، قال: ثم ظهرت الروم بعد، قال: فذلك قوله تعالى {الم غلبت الروم} إلى قوله{يفرح المؤمنون بنصر الله ينصر من يشاء} قال سفيان: سمعت أنهم ظهروا عليهم يوم بدر.