(a) It is narrated from Sayyidah Aishah (may Allah be pleased with her) that the Messenger of Allah (peace and blessings be upon him) said: "Rinsing the mouth and sniffing water into the nose are obligatory in ablution."
(b) Isma'il bin Bishr al-Balkhi also narrated it from Isam in the same way, except that he said: "(They are) part of the ablution without which the prayer is not complete."
(c) Sulayman bin Musa narrates it mursal from the Prophet (peace and blessings be upon him): "Whoever performs ablution, let him rinse his mouth and sniff water into his nose."
Isam is alone in narrating it (this way) and erred in it; the correct version is from Ibn Jurayj, from Sulayman bin Musa, as mursal.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 239