Yahya related to me from Malik from Abu Nuaym that Wahb ibnKaysan said, "The Messenger of Allah, may Allah bless him and granthim peace, was brought food while his stepson Umar ibn Salama was withhim. The Messenger of Allah, may Allah bless him and grant him peace,said to him, 'Say "Bismillah," and eat what is in front of you.' "
USC-MSA web (English) reference: Book 49, Hadith 32
Arabic reference: Book 49, Hadith 1706