Tafsir of Al Imran 3:100

It is said:

Shas ibn Qays, the Jew—who was a man of immense disbelief, fiercely critical of the Muslims, and intensely envious of them—passed by a group of the Ansar from the Aws and Khazraj tribes while they were sitting together talking.

Their unity and gathering, after the enmity that had existed between them during the Age of Ignorance (Jahiliyyah), enraged him. He said, "We will have no stability with them if they remain united."

He ordered a young Jewish man to sit with them, remind them of the Day of Bu'ath (a day when the Aws and Khazraj fought, and the victory went to the Aws), and recite to them some of the poetry composed about it.

The man did so. The people began to dispute, boast, and grow angry, eventually shouting, "To arms! To arms!"

The news reached the Prophet (ﷺ), so he went out to them with those who were with him from the Emigrants (Muhajirun) and the Helpers (Ansar). He said:

"Do you call upon the ways of the Age of Ignorance while I am among you? This is after Allah has honored you with Islam, severed the affairs of the Age of Ignorance from you, and brought your hearts together."

The people realized it was a provocation from Satan and a plot by their enemy. They cast aside their weapons, wept, and embraced one another. Then, they departed with the Messenger of Allah (ﷺ). There was never a day that began more ugly and ended more beautifully than that day.