Reference: Fataawa Ramadhaan – Volume 2, Page 661, Fatwa No.666
Fataawa al-Lajnah ad-Daa.imah lil-Buhooth al-‘Ilmiyyah wal-Iftaa. – Fatwa No.12525
Question: A man used the miswaak (tooth stick) whilst fasting and then assumed that this had broken his fast, so [because of this], he drank and ate afterwards. So, is it obligatory upon him to make up this fast and offer an expiation or [is it sufficient] just to make up the fast?
Response: One who breaks the fast during the day in Ramadhaan [incorrectly] assuming the use of the miswaak breaks the fast, then it is obligatory upon him to make up this fast, repent [to Allaah] and seek forgiveness [from Allaah] at what has come to pass. It is possible that Allaah (Subhaanahu wa-Ta’aala) will forgive him.
And with Allaah lies all the success, and may Allaah send prayers and salutations upon our Prophet Muhammad (sal-Allaahu ‘alayhi wa sallam) and his family and his companions.
