How does Sherlock Holmes know stamford?

Sherlock Holmes and Dr. John Watson are first introduced in the story "A Study in Scarlet". In this story, Watson meets Stamford, an acquaintance of Holmes, at the medical school where Watson is studying. Stamford introduces Holmes to Watson, describing him as a "remarkable man". Holmes and Watson subsequently move into rooms together at 221B Baker Street.

Therefore, Sherlock Holmes does not know Stamford. Instead, Stamford knows Sherlock Holmes.