Is inhaling water during ablution an obligation or a Sunnah, and is it permissible to perform ablution without it during fasting to avoid water entering the interior of the body?

Inhaling water into the nose (istanshaq) is a Sunnah according to the majority of scholars, not an obligation. The ablution (wudu') of one who omits it is valid, but they have forsaken an emphasized Sunnah that the Prophet (peace be upon him) consistently performed. One who intentionally omits it does not incur sin, although consistently neglecting it is considered a deficiency in religious practice and a breach of chivalry. It is recommended to perform istanshaq during ablution without exaggeration while fasting.