The Quran

۞ لَقَد رَضِيَ اللَّهُ عَنِ المُؤمِنينَ إِذ يُبايِعونَكَ تَحتَ الشَّجَرَةِ فَعَلِمَ ما في قُلوبِهِم فَأَنزَلَ السَّكينَةَ عَلَيهِم وَأَثابَهُم فَتحًا قَريبًا

Muhammad Asad

INDEED, well-pleased was God with the believers when they pledged their allegiance unto thee [O Muhammad] under that tree,¹ for He knew what was in their hearts; and so He bestowed inner peace upon them from on high, and rewarded them with [the glad tiding of] a victory soon to come,²

I.e., at Hudaybiyyah (see introductory note).
Most of the commentators assume that this relates to the conquest of Khaybar, which took place a few months after the Truce of Hudaybiyyah. It is probable, however, that the implication is much wider than that - namely, a prophecy of the almost bloodless conquest of Mecca in the year 8 H., the victorious establishment of Islam in all of Arabia and, finally, the tremendous expansion of the Islamic Commonwealth under the Prophet's immediate successors.