Abu Sa'id Khudri reported that Allah's, Messenger (may peace he upon him) forbade from turning the waterskins upside down and drinking from their mouths.
أَنَّهُ قَالَ نَهَى رَسُولُ اللَّهِ ﷺ عَنِ اخْتِنَاثِ الأَسْقِيَةِ أَنْ يُشْرَبَ مِنْ أَفْوَاهِهَا