Source: Michael Fishbane's Biblical Interpretation in Ancient Israel (New York: Oxford, 1988) 110-111.
In my post, Apologetics, Shamar, I write:
"Nehemiah 8:14-15: 'And they found it written in the law, which the LORD had commanded by Moses, that the people of Israel should live in booths during the festival of the seventh month, and that they should publish and proclaim in all their towns and in Jerusalem as follows, 'Go out to the hills and bring branches of olive, wild olive, myrtle, palm, and other leafy trees to make booths, as it is written.'' The problem is that Leviticus 23:39-43 doesn't tell the Israelites to make booths out of those materials; it merely says to rejoice with them. Was Nehemiah's 'law of Moses' different from what we have in our Bibles?"
Fishbane actually comments on this in his book. He disputes that Nehemiah 8:14-15 is a direct exegesis of Leviticus 23:40-42 and Deuteronomy 16:13-15, since there are clear differences. For example, Leviticus 23 talks about four species of vegetation, whereas Nehemiah 8 mentions five. This highlights one major criterion of how Fishbane identifies inner-biblical exegesis: is there similarity between the two passages?
But Fishbane still maintains that there is some exegesis going on here. He doesn't seem to think that Nehemiah is appealing to a different law of Moses from what is in our Bibles. According to him, succoth is from the Hebrew sck, which means "to cover over" with branches. Fishbane states, "On this basis they interpreted the Torah command to dwell in booths as implying that one must dwell in 'branched' shelters--hence the directive to go to the forests and hills to gather representative tree species wherewith to build the booths."
That could be. I don't know. Does Fishbane think Nehemiah wanted the Israelites to rejoice with the tree branches, while making succoth out of them?