Ibn Abbas reported that some Muslims met a person with a small flock of sheep. He said:
As-Salam-o-'Alaikum. They caught hold of him and killed him and took possession of his flock. Then this verse was revealed:" He who meets you and extends you salutations, don't say: You are not a Muslim" (iv. 94). Ibn 'Abbas, however, recited the word as-Salam instead of" as-Salam".
Reference: Sahih Muslim 3025
In-book reference: Book 56, Hadith 26
USC-MSA web (English) reference(deprecated numbering scheme): Book 43, Hadith 7176