(a) Isma'il bin Kathir says: I heard Asim bin Laqit bin Sabirah narrating from his father, who said: I went to the Prophet (peace and blessings be upon him) in the delegation of Banu al-Muntafiq. I said: O Messenger of Allah, tell me about ablution. He (peace and blessings be upon him) said: "Perform ablution completely, pass your fingers between the fingers (and toes), and draw water well into your nose, except when you are fasting."
(b) Sayyiduna Uthman (may Allah be pleased with him) says about the ablution of the Prophet (peace and blessings be upon him): He (peace and blessings be upon him) also passed his fingers between the toes of his feet.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 359