The ruling on one who leaves a spot in his ablution that water did not reach

Question:
He says: Honorable Shaykh, may Allah grant you success. If a person performs wudu' and washes his feet three times, and a spot remains that the water did not reach, is it permissible for him to wash his foot a fourth time, or does he only wash the spot that the water did not wash?

Answer:
If he noticed the omission immediately and his limbs have not dried, he only washes the remaining part, because the continuity has not been broken. However, if his limbs have dried and he did not realize the omission until a long time later, and the continuity has been broken, he must repeat the wudu' from the beginning. This is because the Prophet (peace and blessings be upon him) saw a man who had a spot on his foot that water had not reached, and he ordered him to go back and perfect his wudu'. He did not say to him, "Go back and wash only this spot," but rather, "Perfect your wudu'." So if the continuity is broken, he must start over.