It is narrated from a companion that the Prophet (peace be upon him) saw a man who was praying, and on the back of his foot there was a spot about the size of a dirham where water had not reached. The Prophet (peace be upon him) ordered him to repeat his ablution.
Hadith Referenceمسند احمد / مسند المكيين / 15495
Hadith Gradingحکم دارالسلام:صحيح لغيره، وهذا إسناد ضعيف من أجل بقية ، وهو يدلس تدليس التسوية