[At- Tirmidhi].
An-Nawawi said that the reason why she did this was to preserve the part of the water-skin which the lips of Messenger of Allah (ﷺ) touched, gain blessings from it, and preserve it from dishonor. This narration is understood to show permissibility whereas the previous two narrations are to show what is better and closer to perfection. Allah knows best.
وإنما قطعتها لتحفظ موضع فم رَسُول اللَّهِ ﷺ وتتبرك به وتصونه عن الابتذال. وهذا الحديث محمول على بيان الجواز، والحديثان السابقان لبيان الأفضل والأكمل، والله أعلم.
Reference | : Riyad as-Salihin 763 |
In-book reference | : Book 2, Hadith 37 |