Hisham bin Urwah (may Allah have mercy on him) narrates from his father that the Prophet (peace and blessings be upon him) forbade drinking from the mouth of a waterskin and said: "It becomes spoiled." It has been narrated like this as mursal. As for what has been narrated granting permission in this matter, the reports of prohibition are more authentic in their chains.
Some scholars have interpreted it (the permission) as applying when the waterskin is hanging, so that the creatures of the earth cannot get into it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصداق / 14666