Abu Ghatafan says: I saw Sayyiduna Ibn Abbas (may Allah be pleased with them both) performing ablution; he rinsed his mouth and sniffed water into his nose twice, and he said: The Messenger of Allah (peace and blessings be upon him) said: "When any of you rinses his mouth and sniffs water into his nose, let him do it thoroughly two or three times."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 227