Amr bin Shu'ayb narrates from his grandfather (may Allah be pleased with him) that a Bedouin came to the Prophet (peace and blessings be upon him) and asked him about ablution. He (peace and blessings be upon him) showed him, washing each part three times, then he (peace and blessings be upon him) said: "This is (the method of) ablution. Whoever does more than this has done wrong, or has transgressed and done injustice."
(b) It is narrated from Imam al-Thawri in a connected (mawsul) form. [Hasan. Recorded by al-Tabarani in al-Kabir: 174]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 373
Amr bin Shu'ayb narrates from his grandfather (may Allah be pleased with him) that a man came to the Prophet (peace and blessings be upon him) and said: O Messenger of Allah, how is ablution done? He (peace and blessings be upon him) called for a vessel of water. He washed his palms three times, then washed his face three times, then washed his forearms three times, then wiped his head: he put his two index fingers into his ears, wiped the outer part of his ears with his thumbs and the inner part of his ears with his index fingers. Then he washed his feet three times each, then said: "This is how ablution is done. Whoever does more than this or less has done wrong and done injustice," or he said: "has done injustice and done wrong."
(b) "Does less" possibly means leaving out part of a limb, and "done injustice" means overstepping the limit set by Allah Almighty. And Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 374