Ibn Ma'dan (may Allah have mercy on him) narrates from one of the Companions (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) saw a man praying, and on his foot was a patch the size of a dirham that water had not reached. So the Prophet (peace and blessings be upon him) ordered him "to repeat the ablution and the prayer."
(b) It is like this in the mursal narration, and also in a connected (mawsul) one.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 392