From Berlioz’s Evenings in the Orchestra (1854), translated by Charles E. Roche, pp. 314-315:
Beethoven, when carried away by the subject of Leonora, or
Conjugal Love, saw in it only the sentiments it gave him the
opportunity of expressing, and never took into account the sombre
monotony of the spectacle It presents. The libretto, of French
origin, was first set to music in Paris by Gavaux. Later an Italian
opera was made out of it for Paer, and it was after having heard
in Vienna the music of the latter’s Leonora that Beethoven had
the naive cruelty to say to him: “The subject of your opera pleases
me; I must set it to music.”
It was not the only quip Ludwig had in him. There’s also this, from Anton Schindler’s biography Beethoven as I Knew Him:
On New Year’s Day, 1823, Beethoven, his nephew, and the author were sitting at their noon meal, when the master was handed a New Year’s card from his brother who lived close by. The card was signed, “Johann van Beethoven, Land-owner.” Immediately the master wrote on the back of the card, “Ludwig van Beethoven, Brain-owner,” and returned it to him.