Answer
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.
In this fatwa:
If you forgetfully eat or drink while fasting, you should complete your fast and there is no need for you to redo the fast if you stopped eating as soon as you realized the mistake.
Responding to your question, Sheikh Ahmad Kutty, a senior lecturer and an Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states:
There is no need for you to redo the fast if you stopped eating as soon as you realized the mistake. Abu Hurairah reported that the Prophet (peace be upon him) said, “If someone forgetfully eats or drinks, let him complete his fast, for it is Allah who fed him and offered him drink” (Al-Bukhari)
Allah tells us in the Quran that we are not accountable for our mistakes that are unconscious or accidental:
“Allah does not burden any soul beyond what he or she can endure: in his favor shall be whatever good he does, and against him whatever evil he does. O our Sustainer! Take us not to task if we forget or unwittingly do wrong! ‘O our Sustainer! Lay not upon us a burden such as Thou did lay upon those who lived before us! O our Sustainer! Make us not bear burdens which we have no strength to bear! And efface Thou our sins, and grant us forgiveness, and bestow Thy mercy upon us! Thou art our Lord Supreme: succor us, then, against people who deny the truth!’” (Al-Baqarah 2:286)
Almighty Allah knows best.