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 Prophets 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].
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.