Narrated Sa'eed bin Jubair:
from Ibn 'Abbas, regarding the saying of Allah, Most High: Alif Lam Mim. The Romans have been defeated. In the nearest land (30:1-3)" he said: "Ghulibat wa Ghalabat (defeated and then victorious)." He said: "The idolaters wanted the Persians to be victorious over the Romans because they too were people who worshiped idols, while the Muslims wanted the Romans to be victorious over the Persians because they were people of the Book. This was mentioned to Abu Bakr, so Abu Bakr mentioned that to the Messenger of Allah (ﷺ) and he said: 'They will certainly prevail.' Abu Bakr mentioned that to them, and they said: 'Make a wager between us and you; if we win, we shall get this and that, and if you win, you shall get this or that.' He made the term five years, but they (the Romans) were not victorious. They mentioned that to the Prophet (ﷺ) and he said: "Why did you not make it less (than)" - He (one of the narrators said): I think he said: "ten?" He said: Sa'eed said: "Al-Bid' is what is less than then" - he said: "Afterwards the Romans have been victorious." He said: "That is what Allah Most High said: 'Alif Lam Mim. The Romans have been defeated' up to His saying: 'And on the day, the believers will rejoice - with the help of Allah. He helps whom He wills (30:1-5).' Sufyan said: "I heard that they were victorious over them on the Day of Badr."
English reference: Vol. 5, Book 44, Hadith 3193
Arabic reference: Book 47, Hadith 3497