Valmiki Ramayana states that Ganga is considered sacred because it came by touching Anga (Head) of Lord Shiva:
तत्र ऋषि गण गन्धर्वा वसुधा तल वासिनः १-४३-२६
भव अंग पतितम् तोयम् पवित्रम् इति पस्पृशुः |
And asserting that the water as holy, because it descended touching the body of Shiva, viz., the head of Shiva, the assemblages of sages, gandharva-s, and those that are residents on the plane of earth have sipped that water at that place.
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