All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
The mentioned action does not invalidate fasting because entering the finger into one’s anus does not result in injecting something that could reach one’s stomach. In addition, such an action is not included in the books of religious knowledge as one of the things that invalidates fasting. The nullifiers of fasting have clearly been defined and are known to the people.
Allaah Knows best.