Musnad Ahmad

Sayyiduna Ibn Abbas (may Allah be pleased with them both) used to recite « ﴿الم o غُلِبَتِ الرُّومُ﴾ » in both the well-known and the lesser-known ways, and he used to say that it was always the wish of the polytheists that the Persians would prevail over the Romans, because the Persians were idolaters, while the Muslims wished that the Romans would prevail over the Persians because they were People of the Book. They mentioned this to Sayyiduna Abu Bakr as-Siddiq (may Allah be pleased with him), and Sayyiduna Abu Bakr (may Allah be pleased with him) mentioned it to the Prophet (peace and blessings be upon him). The Prophet (peace and blessings be upon him) said: "The Romans will indeed prevail." Sayyiduna Abu Bakr (may Allah be pleased with him) mentioned this prophecy before the Quraysh. They said: Set a term between us and you; if, according to our view, the Persians prevail, you will give us such and such, and if, according to your view, the Romans prevail, we will give you such and such. Sayyiduna Abu Bakr as-Siddiq (may Allah be pleased with him) set a term of five years, but during that period the Romans did not appear to prevail, so he mentioned this to the Prophet (peace and blessings be upon him). The Prophet (peace and blessings be upon him) said: "Why did you not set a term within ten years?" Sa'id bin Jubayr (may Allah have mercy on him) says that in fact, the word «﴿بِضْعِ﴾» that appears in the Qur'an applies to anything less than ten. Thus, within that period, the Romans prevailed, and this is the background of the opening verses of Surah Ar-Rum.