Sahih Muslim Book 4, Hadith Number 1143.

Chapter : When food is brought before man and he is inclined to take it, he should not say prayer before eating it and undesirability of prayer while feeling the call of nature.

Ibn Suhaib reported: Anas was asked about the garlic; he stated that the Messenger of Allah (may peace be upon him) had said: He who eats of this plant (garlic) should not approach us and pray along with us.