What is the ruling on ablution and prayer for someone who remembered during prayer that he did not perform istinshaq (sniffing water into the nose) during ablution?

Inhaling water into the nose (istinshaq) is obligatory in ablution (wudu'), based on the Prophet's (peace be upon him) saying: "Whoever performs ablution, let him blow his nose (yastanther)," and his saying: "Whoever performs ablution, let him inhale water into his nose (yastanseq)." If one does not inhale water into the nose, their ablution is not valid, and they must repeat both the ablution and the prayer.