I say, and with God's success: Inhaling water through the nose (istinsagh) is a Sunnah in ablution (wudu) and not an obligation as it is in full ablution (ghusl); as narrated from Ibn Abbas (may Allah be pleased with him): "If a man performs ghusl from janabah (sexual impurity) and does not rinse his mouth or inhale water through his nose, he should repeat the ablution, but if he neglects this in wudu, 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 evidence from the narration of Ibn Sirin. And because the mouth is considered both an entrance and an exit: in terms of perception, when the mouth closes and opens. And in terms of ruling, in the swallowing of saliva by the fasting person, its ruling is like that of an entrance since it does not break the fast, and in the entry of something into his mouth, its ruling is like that of an exit since it breaks the fast. Thus, it is considered an entrance in wudu and an exit in ghusl; because what is mentioned in ghusl is in a form of exaggeration: {And if you are in a state of janabah, then purify yourselves} (Al-Ma'idah: 6). And because the obligation in wudu is to wash the face and the inside of the mouth which is not faced, see: Opening the Door of Care 1: 37, Explanation of Al-Wiqayah p.80, and 'Umdat al-Ri'ayah 1: 63, and Allah knows best.