Sahih Muslim Book 4, Hadith Number 1150.

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.

Abu Sa’id al-Khudri reported: The Messenger of Allah (may peace be upon him) along with his Companions happened to pass by a field in which onions were sown. The people stopped there and ate out of that, but some of them did not eat. Then they (Prophet’s Companions) went to him. He (first) called those who had not eaten the onions and kept the others (who had taken onions) waiting till its odour vanished.