Correct spelling, explanation: referred is a correct form of the verb refer which is made up of two Latin roots: -re (back) and -ferre (carry). Thus, this Latin word referre means carry back. Besides, the word referer existed in Old French. And since the 14th century we have had the word refer in English. Taking into account the origin of the word, only the form referred is correct.

Definition of referred:
verb, the regular past verb form of the word refer denoting to direct someone for treatment, aid or information,
My friend referred me to him, saying he was the best specialist in the area.
My doctor referred me to a nephrologist because he suspected a health problem.

Idioms with referred:
1. referred to as — name something or someone in a particular way,
They always referred to her as Professor Smith.
2. cross-referred — direct the reader to another place in the book to get further information,
They read the footnote which cross-referred them to the glossary at the back of the book.

Incorrect spelling, explanation: reffered is incorrect form because we do not double consonant f when we create Past Participle verb. The word comes from the infinitive verb refer in which occurs single f. According to the basic rule, when we build past verb form, we need to double r, not f. As a result, reffered is incorrect.

Incorrect spelling