Sahih Bukhari Volume 007, Book 065, Hadith Number 288.

Narrated By 'Umar bin Abi Salama : I was a boy under the care of Allah's Apostle and my hand used to go around the dish while I was eating. So Allah's Apostle said to me, 'O boy! Mention the Name of Allah and eat with your right hand, and eat of the dish what is nearer to you." Since then I have applied those instructions when eating.