I had never heard that story before...here's something I found about it:
In 1787, Beethoven went to Vienna, a noted musical center, where then Count Waldstein engaged Beethoven was piano teacher and became his friend and patron. Beethoven must have felt a little out of his depth for he was clumsy and stocky; his manners were loutish, his black hair unruly and he habitually wore an expression of surliness on his swarthy face.
It was here that Beethoven met the great Mozart, who was dapper and sophisticated. He received the boy doubtfully, but once Beethoven started playing the piano his talent was evident. "Watch this lad," Mozart reported. "Some day he will force the world to talk about him."
http://www.maurice-abravanel.com/beethoven_more.html