Eukleidés VI.7 (Věta ssu versus Ssu - stř. školy v našem mocnářství - o podobnosti trojúhelníků)
Heath:
If two triangles have one angle equal to one angle, the sides about other angles proportional, and the remaining angles either both less or both not less than a right angle, then the triangles are equiangular and have those angles equal the sides about which are proportional.
http://aleph0.clarku.edu/~djoyce/java/elements/bookVI/propVI5.html