Imam Nawawi has mentioned two reasons from Imam Tirmizi for cutting the mouth portion of the water bag:
1. Tabarrukan (for barakah), because the mubaarak saliva of Sayyidina Rasoolullah sallallahu alaihe wasallam had come in contact with it and she wanted to keep it.
2. She did not want anybody else to drink from the same portion where Sayyidina Rasoolullah sallallahu alaihe wasallam had drank. The reason being respect and courtesy. It will be disrespectful if someone else drank from the same place. There are two things mentioned in this hadith. One is standing and drinking which has already been discussed in Hadith 4. The second is that it is reported in the Bukhari from Sayyidina Anas radiyallahu anhu that it has been prohibited to drink from the mouth of a water bag. Therefore we shall take this hadith to show the permissibility of doing so. It is related in a hadith that a person was drinking from a water bag and a snake came out from it. On hearing this Sayyidina Rasoolullah sallallahu alaihe wasallam forbade the drinking of water direct from a water bag.
According to this hadith, the drinking of water in this manner apparently seems to have taken place in the early period. A special note should be made here. One of the reasons of prohibiting the drinking of water in this manner is that every personâ€™s mouth is not such that if it touches a water bag etc. it will not leave a smell. Certainly there are mouths whose saliva is a cure of diseases, and which is more tastier than everything in the world and creates more happiness than everything else. Therefore the drinking of Sayyidina Rasoolullah sallallahu alaihe wasallam from a water bag cannot be compared with others.