Buy on the rumor, sell on the news.
While trying to find out where that came from I found:
"Buy on the rumor, sell on the news," is an old saying on Wall Street that dates back to the 1920s.
Seems right, it was an era when the trading floor (w/o computer monitors) would be abuzz w/ half-heard stories that couldn't be confirmed until the daily news paper would hit the streets the next day. Nowadays we're in a situation where maybe we should say "day-trade on the tweet, post derivative on the talk-radio".
Or something like that...