The Rigveda, a collection of sacred texts that has formed the base of the Hindu religion and much Indian culture, was verbally composed (originally in archaic Sanskrit) as early as 1700 BC.
The Rigveda represents the earliest known writings by the Indo-Aryan peoples, though no document survives containing the original Sanskrit text -- in fact, nothing from anything near that period. The Indus Script had died out by the this time, and writing did not return until around the 3rd century BC, until which (at the very earliest) the Vedas must have been transmitted from generation to generation orally.
Now, the Indus Valley civilisation, which is commonly thought to have died out either before or during the invasion/migration of the Indo-Aryan people into north-west India, possessed its own entirely different language (non Indo-European, possibly Dravidian) and possibly writing system (Harappan script), although linguists are yet to determine if Harappan symbols actually represented a language. If the Harappan script ever gets deciphered and translated, then literary evidence of pre-Aryan civilisation, going back perhaps as far as 2600 BC, may be revealed.
As it stands, the Rigveda is the oldest known literary reference to life in India. The problem is that all nearby civilisations were illiterate in the technical sense at this time (excepting the undecipherable Harappan script). Chinese writing was only developed at earliest from 1600 BC. While Elamite and Sumerian scripts in the Middle East would have been long established by 1700 BC, the contact between these civilisations and the Ancient Indo-Aryan civilisation has not been recorded in history, unfortunately.