Yahoo is based on Bing. As reported above, Microsoft invested in Yahoo! many years ago.
Search is core to a lot of what Microsoft does, so I can't see them giving it up. If anything, they continue to invest in it. If you take a look at a current Windows Phone 8.1, you'll see how far they've pushed and integrated it with Cortana.
Of course, nothing is impossible, but I'd say it is 98% unlikely that Yahoo would be buying Bing unless Microsoft were to completely own Yahoo!