This old-time comic strip has a professor in front of pupils, trying to prove the Pythagorean Theorem on a chalk board....
Professor: ...Und nun will ich Ihnen diesen Lehrsatz jetzt auch beweisen.
Junge: Wozu beweisen, Herr Professor? Ich glaub' es Ihnen so.
"Fester Glauben" in Max & Moritz, 1880
English translation by H. Arthur Klein:
Teacher: ...and now I want to prove this theorem.
Pupil: Why bother to prove it teacher? I take your word for it.
"Firm Faith" in Max & Moritz, 1962
Source: Source: A neighbor A.D. who was raised as a kid reading this German comic strip