Reference for Pesachim 33:3

רב אמר אישתבש כהני רביעי בקדש בעא מינייהו ואמרו ליה טהור

[To turn to] the main text: 'Rab said: The priests erred; but Samuel maintained, The priests did not err'.' Rab said, The priests erred'; he asked them about a fourth degree in respect of holy foodstuffs, and they answered him that it was clean.'

Jastrow