Ar-Rum 30:2

When the Sasanian Persian army inflicted a heavy defeat on Byzantium, the Meccan polytheists rejoiced, for they identified themselves with the scriptureless Persians and the Muslims with the Byzantines, a people of the Book. Thereupon the verses were revealed: "The Romans have been defeated in the nearest land; but after their defeat they will be victorious within a few years (bid' sinin)." As narrated in Tirmidhi, Abu Bakr made a wager with the polytheists that Byzantium would prevail within the stated period; the foretold victory came to pass, to the joy of the believers.

Tirmizî 3193

Ar-Rum is Meccan. It was revealed after Sasanian Persia inflicted a heavy defeat on Byzantium (roughly 613-619) and foretold that the Romans would prevail within a few years. The fulfilment of this prediction, which reports associate with the years around Badr, was counted as evidence of the truth of revelation.

Klasik tefsir mukaddimeleri; Tirmizî rivayetleri