A man came to the Prophet ﷺ having performed ablution, but he had left a spot on his foot the size of a fingernail untouched. The Messenger of Allah ﷺ said to him, "Go back and perform your ablution properly."
حدثنا هارون قال أبو عبد الرحمن وسمعته أنا من هارون غير مرة حدثنا عبد الله بن وهب قال حدثني جرير بن حازم أنه سمع قتادة بن دعامة
أن رجلا جاء إلى النبي ﷺ قد توضأ وترك على قدمه مثل موضع الظفر فقال له رسول الله ﷺ ارجع فأحسن وضوءك
Its isnad is sahih. It is found in Muslim, from Jabir, 1/215 no. 243, in the chapter on Tahara, the necessity of covering every part of the area being purified, and in Abu Dawud, 1/24 no. 173, similarly, and in Ibn Majah, 1/218 no. 665, "Whoever performs wudu but leaves out a spot," both of the latter two from Anas.