Full Idea
The thinking behind Pythagorean philosophy is that people behave in an absurd fashion if they try to find any source for the good other than the gods.
Gist of Idea
Pythagoreans believe it is absurd to seek for goodness anywhere except with the gods
Source
Iamblichus (Life of Pythagoras [c.290], 137)
Book Reference
'The First Philosophers', ed/tr. Waterfield,Robin [OUP 2000], p.98