I said, "Messenger of Allah, tell me about ablution."He said, "Perform the ablution thoroughly, run your fingers between your toes, and inhale water deeply into your nose unless you are fasting."
حدثنا عبد الوهاب بن عبد الحكم البغدادي الوراق وأبو عمار الحسين بن حريث قالا حدثنا يحيى بن سليم حدثني إسمعيل بن كثير قال سمعت
قلت يا رسول الله أخبرني عن الوضوء قال أسبغ الوضوء وخلل بين الأصابع وبالغ في الاستنشاق إلا أن تكون صائما
[Its isnad is sahih] Abu Dawud narrated it in the Book of Fasting, chapter "The Fasting Person May Have Water Poured on Him from Thirst, and He Should Be Thorough in Sniffing Water Up the Nose," hadith no. 2366, from the hadith of Yahya bin Salim through this route. Ibn Khuzayma graded it sahih, hadith nos. 150, 168, as did Ibn Hibban, hadith no. 159, and Hakim, 1/147, 148, and Dhahabi and others.