Riyad Us-Saliheen (Gardens of the Righteous)
Chapter 112
Undesirability of Drinking directly from the mouth of a Water-Skin
762. Abu Sa`id Al-Khudri (May Allah be pleased with him) reported: Messenger of Allah (PBUH) forbade turning the water-skin upside down and drinking directly from its mouth.
[Al-Bukhari and Muslim].
Commentary: Messenger of Allah (PBUH) has forbidden us to drink water from the mouth of a water-skin or something similar to it because some harmful insect in the water-skin may flow into the stomach. So, instead of drinking direct from the mouth of the water-skin, it is better to pour it into a drinking vessel in order to shun any possible harm.
763. Abu Hurairah (May Allah be pleased with him) reported: Messenger of Allah (PBUH) prohibited drinking directly out of the mouth of a water-skin.
[Al-Bukhari and Muslim].
Commentary: This Hadith and the preceding one are complementary to each other.
764. Umm Thabit Kabshah (May Allah be pleased with her), daughter of Thabit (May Allah be pleased with him) and the sister of Hassan bin Thabit, the Prophet's poet) reported: Messenger of Allah (PBUH) visited me and drank some water from a hanging water-skin while he was in a standing posture. So, I stood up and cut off the mouth (of that water-skin). An-Nawawi said that the reason why she did this was to keep to receive benediction that part of the water-skin which the lips of Messenger of Allah (PBUH) touched.
[At-Tirmidhi].
Commentary: The first two Ahadith forbid us from drinking water out of the mouth of a water-skin, whereas, once Messenger of Allah (PBUH) himself had done so. This shows that not to drink water by this way is also approved. However, if needed, it is allowable. For this reason Imam An-Nawawi clarified that to drink water out of the mouth of the water-skin is conditionally permissible and not forbidden. But to eschew it is desirable. Likewise, to receive benediction from the Prophet's left-overs is permissible as long as this does not take the form of worship and glorification.