A little history of a great line I first heard in a trailer for a new Christopher Robin film.
People say nothing is impossible, but I do nothing every day
In conclusion, this joke was in circulation by 1906 when it was printed in a humor book authored by “Theodor Rosyfelt”. The phraseology changed as it was transmitted across decades. In modern times it has been connected to two iconic figures: Alfred E. Neuman and Winnie-the-Pooh. QI has not found any substantive evidence that A.A. Milne used the expression.
It remains a very Winnie-the-Pooh thing to say, though. Except in East Germany.