It is narrated from Hazrat Anas (may Allah be pleased with him) that a man came to the Prophet (peace and blessings be upon him). He had performed ablution, but a spot equal to the size of a nail on his foot was left dry. The Prophet (peace and blessings be upon him) said to him, "Go back and perform ablution properly."
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 12487
Hadith Gradingحکم دارالسلام:إسناده صحيح، وله شاهد من حديث عمر بن الخطاب عند مسلم: 243 سلف برقم: 134