Early formal Reformed attestation to Sola Scriptura (original source here):

The Genevan Confession was credited to John Calvin in 1536 by Beza who said Calvin wrote it as a formula of Christian doctrine suited to the church at Geneva. More recent scholarship attributes it to William Farel but in all likelihood Calvin did have considerable influence on the document. Indeed the records of the Senate at Geneva indicate that the confession was presented by both Farel and Calvin to the magistrates who received it and set it aside for more detailed examination.

I. The Word of God

First we affirm that we desire to follow Scripture alone as rule of faith and religion, without mixing with it any other thing which might be devised by the opinion of men apart from the Word of God, and without wishing to accept for our spiritual government any other doctrine than what is conveyed to us by the same Word without addition or diminution, acccording to the command of our Lord.