The Messenger of Allah (may Allah's peace and blessings be upon him) forbade turning the waterskins upside down and drinking from their mouths
Authentic hadithعن أبي سعيدٍ الخُدْرِيِّ رضي الله عنه قال: نهى رسولُ اللهِ صلى الله عليه وسلم عن اخْتِنَاثِ الأَسْقِيَةِ.
Abu Sa‘īd al-Khudri (may Allah be pleased with him) reported: The Messenger of Allah (may Allah's peace and blessings be upon him) forbade turning the waterskins upside down and drinking from their mouths.
Explanation
The Prophet (may Allah's peace and blessings be upon him) forbade turning the waterskins upside down and then drinking from their mouths, because they may contain harmful things that could enter one's stomach and harm him.
Source · Narrated by Bukhari & Muslim
From the Hadith Encyclopedia (hadeethenc.com).