In research by Carol Dweck, et al. 5th graders were given a short written test, then received either of the following responses:

"You did really well; you must be very smart."

OR

"You did really well; you must have worked very hard."

The students who received the response, "You did really well; you must have worked very hard." were more likely to choose more challenging tasks when the researchers next offered, and they were much more likely to say that they enjoyed the tasks. They also did better on the subsequent test administered at the end of the study.

Mindsetonline.com

Mueller, C. M., & Dweck, C. S. (1998). Intelligence praise can undermine motivation and performance. Journal of Personality and Social Psychology, 75, 33-52.