What is the true definition of eating and drinking which invalidate one’s fasting? – IslamonLine.net – Ask The Scholar
Wa`alaykum as-salamu wa rahmatullahi wa barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Dear brother in Islam, we commend your eagerness to become well acquainted with Islam and its teachings, which is the way Allah has chosen for the welfare of His servants.
A Muslim should be aware of the things that invalidate his fasting in order to avoid them and thus safeguard his worship. Special concern has risen in modern age particularly regarding some medical tools or drugs used during fasting, which necessitates some kind of ijtihad.
In response to the question, Sheikh `Atiyyah Saqr, former head of Al-Azhar Fatwa Committee, states:
via What Invalidates the Fast – IslamonLine.net – Ask The Scholar.

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