The Messenger of Allah (may Allah's peace and blessings be upon him) forbade turning the waterskins upside down and drinking from their mouths

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.
Authentic hadith - Narrated by Bukhari & Muslim

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.

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