Rinsing the mouth is a Sunnah in ablution and not an obligation as it is in bathing; as narrated from Ibn Abbas (may Allah be pleased with him): "If a man bathes from sexual impurity and does not rinse his mouth or sniff water, he should repeat the ablution, but if he leaves that in ablution, he does not have to repeat it." In the sources 1: 13, Al-Tahanawi said in I'laa al-Sunan 1: 183: The hadith is good and acceptable for argument, and it has a correct supporting narration from the mursal of Ibn Sirin. This is because the mouth is considered both an entrance and an exit: in a tangible sense when the mouth closes and opens, and in a legal sense regarding the swallowing of saliva by a fasting person, as it is considered an entrance since it does not break the fast, and regarding the entry of something into his mouth, it is considered an exit since it breaks the fast. Thus, it is treated as an entrance in ablution and an exit in bathing; because the wording in bathing is in the form of exaggeration: {And if you are in a state of janabah, then purify yourselves} (Al-Ma'idah: 6). And because the obligatory act in ablution is washing the face and the inside of the mouth, which is not faced. See: Fath Bab al-'Inayah 1: 37, Sharh al-Wiqayah p. 80, and Umdat al-Ri'ayah 1: 63, and Allah knows best.