Narrated Abdullah ibn AbuBakr: Abdullah ibn AbuBakr and some children of Muhammad ibn Maslamah said: There remained some people of Khaybar and they confined themselves to the fortresses. They asked the Messenger of Allah (ﷺ) to protect their lives and let them go. He did so. The people of Fadak heard this; they also adopted a similar way. (Fadak) was, therefore, exclusively reserved for the Messenger of Allah (ﷺ), for it was not captured by the expedition of cavalry and camelry.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا حُسَيْنُ بْنُ عَلِيٍّ الْعِجْلِيُّ، حَدَّثَنَا يَحْيَى، - يَعْنِي ابْنَ آدَمَ - حَدَّثَنَا ابْنُ أَبِي زَائِدَةَ، عَنْ مُحَمَّدِ بْنِ إِسْحَاقَ، عَنِ الزُّهْرِيِّ، وَعَبْدِ اللَّهِ بْنِ أَبِي بَكْرٍ، وَبَعْضِ، وَلَدِ مُحَمَّدِ بْنِ مَسْلَمَةَ قَالُوا بَقِيَتْ بَقِيَّةٌ مِنْ أَهْلِ خَيْبَرَ تَحَصَّنُوا فَسَأَلُوا رَسُولَ اللَّهِ صلى الله عليه وسلم أَنْ يَحْقِنَ دِمَاءَهُمْ وَيُسَيِّرَهُمْ فَفَعَلَ فَسَمِعَ بِذَلِكَ أَهْلُ فَدَكَ فَنَزَلُوا عَلَى مِثْلِ ذَلِكَ فَكَانَتْ لِرَسُولِ اللَّهِ صلى الله عليه وسلم خَاصَّةً لأَنَّهُ لَمْ يُوجِفْ عَلَيْهَا بِخَيْلٍ وَلاَ رِكَابٍ .
sunnah.com