Early Sanskrit references were found in the Vedas. Vedas are the earliest know Sanskrit literature text in history.
Writing system/technology wasn't developed during the time of Vedas. The writing came in a much later part of history. Knowledge used to be passed on using an oral tradition that is the recitation of Vedas and other texts.
Panini is the one who consolidated grammar rules through his text - Ashtadhyayi. It is considered the finest Sanskrit grammar text till now.
So no one person wrote the Sanskrit language. It was there since the Vedic period and Panini made notable contributions to it which can be seen even today!
