It is narrated from Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) "forbade drinking water from the mouths of waterskins."
Al-Asma'i said: "Ikhtinath" means folding back the mouth of the waterskin and drinking from it, or turning the mouth of the waterskin upward and drinking from it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصداق / 14661