Sunan an-Nasa'i, Volume 4, Book 35, Hadith 3841 (DarusSalam) ⓘ
It was narrated that Ibn 'Abbas said: "The Messenger of Allah passed by a man who was leading another man by a rope. The Prophet took it, and cut it, and he said: 'It is a vow.'"
أَخْبَرَنَا مُحَمَّدُ بْنُ عَبْدِ الأَعْلَى، قَالَ حَدَّثَنَا خَالِدٌ، عَنِ ابْنِ جُرَيْجٍ، قَالَ حَدَّثَنِي سُلَيْمَانُ الأَحْوَلُ، عَنْ طَاوُسٍ، عَنِ ابْنِ عَبَّاسٍ، قَالَ مَرَّ رَسُولُ اللَّهِ صلى الله عليه وسلم بِرَجُلٍ يَقُودُ رَجُلاً فِي قَرَنٍ فَتَنَاوَلَهُ النَّبِيُّ صلى الله عليه وسلم فَقَطَعَهُ قَالَ إِنَّهُ نَذْرٌ .
also referenced under: Hadith 3810 (Reference) & Book 35, Hadith 50 (In-Book)