diff --git a/code/math.f90 b/code/math.f90
index 315da2642..e62fe60c3 100644
--- a/code/math.f90
+++ b/code/math.f90
@@ -1181,6 +1181,9 @@ end function math_Voigt66to3333
 
 !--------------------------------------------------------------------------------------------------
 !> @brief random quaternion
+! http://math.stackexchange.com/questions/131336/uniform-random-quaternion-in-a-restricted-angle-range
+! K. Shoemake. Uniform random rotations. In D. Kirk, editor, Graphics Gems III, pages 124-132. 
+! Academic, New York, 1992.
 !--------------------------------------------------------------------------------------------------
 function math_qRand()