Sayyiduna Abu Hurairah (may Allah be pleased with him) said: The Messenger of Allah (peace and blessings be upon him) said: "When any one of you eats or drinks forgetfully while fasting, then remembers, he should complete his fast, for indeed it is Allah who has fed him and given him drink." Imam Darimi (may Allah have mercy on him) said: The people of Hijaz say that in such a case, the fast must be made up, but I say that it does not have to be made up.
Hadith Referenceسنن دارمي / من كتاب الصوم / 1765
Hadith Gradingتحقیق (حسین سلیم أسد الدارانی):إسناده جيد إذا ثبتت الصحية لعم الحارث وهو: عياض بن عبد الله بن سعد بن أبي ذئاب فقد ذكره ابن مندة وأبو نعيم في الصحابة وإلا فهو مجهول والحديث متفق عليه، [مكتبه الشامله نمبر: 1768]
Hadith Takhrijاس روایت کی تخریج اوپر گذر چکی ہے۔
Brief Explanation
(Commentary on Hadiths 1763 to 1765)
From these hadiths, it is established that if someone, while fasting, forgetfully eats or drinks something, then his fast remains valid; neither is there any making up (qada) upon him nor any expiation (kaffarah). This is the correct position.