Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.030B.

Abu Hurairah reported that the Prophet, upon whom be peace, said, “When one of you performs ablution, he should sniff water up his nostrils and then blow it out.” (Related by al-Bukhari, Muslim and Abu Dawud.)

The sunnah is to put the water into the nostrils with the right hand and blow it out with the left. ‘Ali once called for water for ablution, rinsed his mouth, sniffed up water into his nostrils and blew it out with his left hand. He did that three times and then said, “That is how the Prophet, upon whom be peace, would purify himself.” (Related by Ahmad and an-Nasa’i.)

This sunnah is fulfilled by putting water into the mouth and nostrils in any way. The practice of the Prophet was to do both acts at the same time. ‘Abdullah ibn Zaid said, “The Prophet would rinse his mouth and nose with just one hand (at one time, together). He did that three times.” In one narration it says, “He would rinse his mouth and nose with three scoops of water.” (Related by al-Bukhari and Muslim.) It is also sunnah to be plentiful (with water) while performing this sunnah, except if one is fasting. Laqit asked the Prophet, “Inform me about your ablution.” He replied, “Complete and perfect the ablution and (put water) between your fingers. Use lots of water while sniffing it up your nostrils, unless you are fasting.” (Related by “the five.” At-Tirmizhi said it is sahih.)