Is it obligatory to leave the prayer and repeat it when one's ablution is frequently nullified due to stomach upsets during prayer, and does the ruling differ between fasting in Ramadan and voluntary fasting?

If your stomach upsets are continuous due to fasting, such that you do not find a time to perform prayer without passing wind, then you are afflicted with wind incontinence. In this case, you must perform ablution after the prayer time has begun and pray, even if wind is emitted, and this does not prevent you from observing obligatory fasts. As for supererogatory fasts, if they cause the purity for prayer to be invalidated, then it is better not to observe them.

However, if you can find a time to perform ablution and prayer without passing wind, then you are not considered to have a continuous nullifier of ablution. If wind is emitted during prayer, you must repeat the prayer, but you should not abandon either obligatory or supererogatory fasts.