Where in the Bible does it refer to Isaiah being sawn in half while hiding in a tree?

That’s not in the Bible, but it is contained in some ancient texts.

There is a tradition reported in the Martyrdom of Isaiah (a Christian text from around AD100 which expands on 2 Kings 21) that Isaiah was condemned to death by King Manasseh. Although he hid in a tree, he was found and the tree with Isaiah inside was sawn in half. A similar tradition is recorded in Lives of the Prophets (another Christian text from around AD100). The method of Isaiah’s death (sawn in half) is also supported by the Jewish texts known as the Babylonian Talmud and the Jerusalem Talmud (both written after AD200).

The only possible biblical reference to these gruesome events is in Hebrews 11:37 which describes faithful people from the past:

They were stoned, they were sawn in two, they were killed with the sword. They went about in skins of sheep and goats, destitute, afflicted, mistreated–