Specifically, it’s from:
“The Unanimous
Declaration of Independence
made by the
Delegates of the People of Texas
in General Convention
at the town of Washington
on the 2nd day of March 1836.”
http://www.lsjunction.com/docs/tdoi.htm
Written literally overnight in bitter cold at Washington-on-the-Brazos.
Our state Constitution goes one step further:
All political power is inherent in the people, and all free governments are founded on their authority, and instituted for their benefit. The faith of the people of Texas stands pledged to the preservation of a republican form of government, and, subject to this limitation only, they have at all times the inalienable right to alter, reform or abolish their government in such manner as they may think expedient.
Article 1, Section 2, Texas State Constitution.
Don't mess with Texas. God bless Texas.
The the battle begin, one way or the other.