YALMIP is free of charge to use and is openly distributed, but note that
1. Copyright owned by Johan Löfberg.
2. YALMIP must be referenced when used in a published work (give me some credit for saving your valuable time!)
3. YALMIP, or forks or versions of YALMIP, may not be re-distributed as a part of a commercial product unless agreed upon with the copyright owner (if you make money from YALMIP, let me in first!)
4. YALMIP is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE (if your satellite crash or you fail your Phd due to a bug in YALMIP, your loss!).
5. Forks or versions of YALMIP must include, and follow, this license in any distribution.
Something like this could be a suitable general reference (to write ö, use \"{o})
YALMIP : A Toolbox for Modeling and Optimization in MATLAB. J. Löfberg. In Proceedings of the CACSD Conference, Taipei, Taiwan, 2004. (URL) (BIB)
If you use the sum-of-squares module, a reference to the associated paper might be in place
Pre- and post-processing sum-of-squares programs in practice. J. Löfberg. IEEE Transactions on Automatic Control, 54(5):1007-1011, 2009. (URL) (PDF) (BIB)
Similarily, if you rely on the automatic dualization,
Dualize it: software for automatic primal and dual conversions of conic programs. J. Löfberg. Optimization Methods and Software, 24:313 - 325, 2009. (URL) (PDF) (BIB)
And finally the robust optimization module
Automatic robust convex programming. J. Löfberg. Optimization methods and software, 27(1):115-129, 2012. (URL) (BIB)