Reference: Fataawa Ramadhaan – Volume 1, Page 224, Fatwa No.169
Question: I fasted a day but I had forgotten during the morning and had eaten. Then, I continued with my fast. Is there any sin upon me?
Response: One who forgetfully eats or drinks whilst fasting should complete his fast for, certainly, it was Allaah who provided this food and drink for him as has been mentioned in the Hadeeth. Allaah has forgiven the mistake and forgetfulness and does not take anyone to account until they have done an act intentionally.