Correct spelling, explanation: referring, which is the present participle of refer, derives from referre, which was a Latin word meaning to carry back. It was a combination of -re and ferre, which corresponds to back and bring. Later, referring appeared as referer in Old French, and then, in Middle English, it started to function in the English language.

Definition of referring:
1. verb (present participle form) – mentioning someone or something; relating to someone or something.
It’s a good presentation of the topic, but while speaking she’s referring too much to her notes.
Jane is always referring only to her father as her life mentor, which for me as her mother is quite annoying.

Collocations with referring:
A few common collocations with referring are: referring commonly, specifically referring, and referring to an expert.
Referring too commonly to only one author in your thesis may make it look like plagiarism.
There might be a hundred students in the lecture, but I know Miss Jackson will be specifically referring to me as she literally hates me.
Referring to an expert is the best way to make your article sound reliable and serious.

Incorrect spelling, explanation: according to a spelling rule if a word ends with a consonant that is preceded by a vowel, we need to double that consonant when we want to add e.g. the -ing ending. Therefore, despite the fact the base form of the verb is refer, after we add the -ing, it becomes referring. A number of English users forget about this rule and they spell referring as refering, which is incorrect.