Juz' 21: Suurah Ar-Ruum, Aayah 1-6
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.الم
.غُلِبَتِ الرُّومُ
.فِي أَدْنَى الْأَرْضِ وَهُم مِّن بَعْدِ غَلَبِهِمْ سَيَغْلِبُونَ
.فِي بِضْعِ سِنِينَ لِلَّهِ الْأَمْرُ مِن قَبْلُ وَمِن بَعْدُ وَيَوْمَئِذٍ يَفْرَحُ الْمُؤْمِنُونَ
.بِنَصْرِ اللَّهِ يَنصُرُ مَن يَشَاء وَهُوَ الْعَزِيزُ الرَّحِيمُ
.وَعْدَ اللَّهِ لَا يُخْلِفُ اللَّهُ وَعْدَهُ وَلَكِنَّ أَكْثَرَ النَّاسِ لَا يَعْلَمُونَ
Alif, Lam, Meem.
The Byzantines have been defeated
In the nearest land. But they, after their defeat, will overcome.
Within three to nine years. To Allah belongs the command before and after. And that day the believers will rejoice
In the victory of Allah . He gives victory to whom He wills, and He is the Exalted in Might, the Merciful.
[It is] the promise of Allah . Allah does not fail in His promise, but most of the people do not know. [30:1-6]
The story behind these verses are a must to read/listen to in order to fully grasp their greatness. In short, the Byzantines were defeated by the Persians and were eventually encircled in Constantine (todays Istanbul). The Persians, however, did not manage to enter the city. Ceasar of the Byzantines promised the Persians wealth and possession far exceeding what they had. Of course, there were no banks at that time, instead the wealth used to be buried in the ground.
The Persians accepted the promise and made a way for him and and some of his army to bring the gold and silver and whatnot. The Persians were still surrounding the city with almost all the soldiers in the state. Ceaser, instead of bringing what he promised, attacked the cities of the Persians, killed the men and captured the women and children. He continued until he reached the capital and captured the womenfolk of Kisra (the Persian king) himself! He put the son of Kisra on a donkey sending him to his father with a letter saying "there is the wealth I promised".
The Persians, no matter how hard they tried, were not able to invade Constantine. They came to the agreement that Ceaser and his little army would not see their encircled companions again and cut off the only road leading to the city. Ceaser did a manouvre completely destroying the Persian defence; he went with his army to the lake and filled the boats with different materials. He later returned with his army but left the boats with the materials in the lake. Kisra, having heard the manouvre, went with his army to the Lake, and thus Ceaser arrived in Constantine without a fight.
This is what these verses refer to.
Notice how the Qur'aan says "in three to nine years". The non-muslims of Quraysh found it far-fetched that the Byzantines, having suffered so manhy losses and were completely encircled by a far superior enemy, that they would ever rise again let alone doing it in 3 to 9 years (as the word Bid' means in arabic). They made a deal with Abubakr that, since the Qur'aan said within 3-9 years, let's agree on six years. Abubakar agreed and lost the deal. He was told be the Sahaaba "why did you narrow it down to six years when Allah said nine?". The Hadith concerning this issue can be found in Tirmidhi (3194) wherein he also mentions that...
They were defeated within nine years. And upon that, alot of people entered Islam. How couldn't they, when the prophecy that seemed so far-fetched happened exactly as was foretold by the Qur'aan? Subxaan Allah. Take this as a lesson that it is only Allahs plan that will become reality. So make him your ally and your reliance. Indeed, only Him is the Possessor of all might.
[The story can be found in Tafsiir Ibn Kathir]