The phrase "as it is written" or "what is written" occurs frequently in the New Testament and usually refers to commands or prophecies that were written in the Hebrew scriptures.
* Sometimes "as it is written" refers to what was written in the Law of Moses.
* Other times it is a quote from what one of the prophets wrote in the Old Testament.
* This could be translated: "as it is written in the Law of Moses" or "as the prophets wrote long ago" or "what it says in God's laws that Moses wrote down long ago".
* Another option is to keep "It is written" and give a footnote that explains what this means.