Megasthenes

He described India in his book Indica, which is now lost, but has been partially reconstructed from literary fragments found in later authors that quoted his work.

[2] Megasthenes was then an ambassador for Seleucid king Seleucus I Nicator and to the court of the Mauryan Emperor Chandragupta Maurya in Pataliputra (modern Patna).

Dating for his journey to the Mauryan court is uncertain; Seleucus I reigned from 305 to 281 BCE for the loose range of years that Megasthenes' mission might have begun.

[3][4] Megasthenes was a Greek ambassador of Seleucus I Nicator in the court of Chandragupta Maurya.

[10] He appears to have passed through the Punjab region in north-western India, as he provides a detailed account of the rivers in this area.