While we don't have any recordings of his actual speech, it's generally accepted that he and his contemporaries in Iceland and Greenland spoke Old Norse. This is based on:
* Historical records: The sagas (stories) about Leif Erikson and other Vikings were written in Old Norse, providing valuable insight into their language and culture.
* Linguistic evidence: The descendants of the Viking settlements in Greenland still speak a language called "Greenlandic" which evolved from Old Norse.
* Archaeological findings: Runestones and other artifacts from Viking settlements provide further evidence for the use of Old Norse.
It's important to note that Old Norse was not a single, monolithic language. There were regional dialects and variations, and the language spoken by Leif Erikson may have differed slightly from the language spoken in other parts of Scandinavia. However, it's safe to assume that he spoke a form of Old Norse.