Must prayers that were performed facing a wrong Qibla direction be repeated after the discovery of the correct direction?

If the informant is a Muslim not known for transgression, then there is no harm in relying on his report. If the deviation from the Qiblah is minor, it does not invalidate the prayer, and thus no re-performance is necessary. However, if it is significant, such as turning one's back to the Qiblah, or facing it to the right or left, then the previous prayers must be re-performed.