Correct spelling, explanation: we double the r in this verb because in the case of the last syllable stressed in the two-syllable word we double the consonant ending the word when we add -ed to it. Refer stresses the second syllable and it also ends with a consonant-vowel-consonant combination, so the correct past form will be referred. Refered is incorrect as it doesn’t double the last r.

Definition of referred:
verb, past form (regular) of refer; to mention something or somebody the particular way; also, in medicine to send a patient to treatment
We referred to it as “the Register” in this document.
Jack has been referred for MRI.

Incorrect spelling, explanation: in such case, when the verb ends with a consonant-vowel-consonant combination and the last syllable is stressed (reFER), we double the last letter. Thus refered is incorrect and we should spell this verb as referred. Refer + -ed = referred (doubled r).

Incorrect spelling