Musannaf Ibn Abi Shayba 1:181

Ibn ‘Ulayyah narrated to us, from Dawud, from al-Sha’bi, who said: It is only the wiping of the two feet. Do you not see that what was [originally] subject to washing, Tayammum was placed upon it, and what was [originally] subject to wiping was neglected, so Tayammum was not placed upon it?