First heard of this from Dr. Tong on 1John 5:9 (The testimony of God greater than men):
Then I fact checked:
1787, Beethoven did indeed went to see Mozart for tutelage. Beethoven was 17, using Max Franz's recommendation letter to gain Mozart's audience. Mozart was tired indeed, wasn't interested to listen to Beethoven. Beethoven played Mozart's Piano Concerto No. 24 in C minor, but Mozart stopped him to have Beethoven play his own piece.
Then Mozart indeed proclaimed to the public, but mainly his wife Constanze: "Stanzi, Stanzi," he said, pointing back into the music room, "Watch out for that boy. One day he will give the world something to talk about."
Unfortunately, urgent news of Beethoven's mother's illness prevented Beethoven to be tutored by Mozart despite Mozart's agreement to take him on as a student.