Final answer:
The Old Testament of the Bible is the book where the first written mention of grass is found, predating other texts like the Dead Sea Scrolls.
Step-by-step explanation:
The first written mention of grass is found in the Old Testament of the Bible. This is one of the oldest texts known to contain references to various plants, including grass. The Dead Sea Scrolls, which date back to between 100 BC and AD 50, also contain many ancient texts, but the specific mention of grass as the question asks for is from the Old Testament.