Mukhtasar Muslim (Abridged) 134 — Whoever leaves a part unwashed

It is narrated from Sayyiduna Jabir (may Allah be pleased with him) that Sayyiduna Umar bin Khattab (may Allah be pleased with him) told me that a man performed ablution and left a spot on his foot, the size of a fingernail, dry. The Messenger of Allah (peace and blessings be upon him) saw him and said, "Go and perform ablution properly." So he went back, then returned and offered prayer.