Sahih Muslim Book 4, Hadith Number 1151.

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.

Ma’dan b. Talha reported: ‘Umar b. Khattab, delivered the Friday sermon and he made a mention of the Apostle of Allah (may peace be upon him) and Abu Bakr. He (further) said: I saw in a dream that a cock pecked me twice, and I perceive that my death is near. Some people have suggested me to appoint my successor. And Allah would not destroy His religion. His caliphate and that with which He sent His Apostle (may peace be upon him) If death approaches me soon, the (issue) of Caliphate (would be decided) by the consent of these six men with whom the Messenger of Allah (may peace be upon him) remained well pleased till his death. And I know fully well that some people would blame me that I killed with these very hands of mine some persons who apparently professed (Islam). And if they do this (blame me) they are the enemies of Allah, and are non-believers and have gone astray. And I leave not after me anything which to my mind seems more important than Kalala. And I never turned towards the Messenger of Allah (may peace be upon him) (for guidance) more often than this Kalala, and he (the Holy Prophet) was not annoyed with me on any other (issue) than this: (And he was so perturbed) that he struck his fingers on my chest and said: Does this verse that is at the end of Surat al-Nisa’, which was revealed in the hot season not suffice you? And if I live longer I would decide this (problem so clearly) that one who reads the Qur’an, or one who does not read it, would be able to take (correct),decisions (under its light). He (‘Umar) further said: Allah ! I call You witness on these governors of lands, that I sent them to (the peoples of these lands) so that they should administer justice amongst them, teach them their religion and the Sunnah of the Apostle of Allah (may peace be upon him), and distribute amongst them the spoils of war and refer to me that which they find difficult to perform. O people, you eat these two plants and these are onions and garlic and I find them nothing but repugnant for I saw that when the Messenger of Allah (may peace be upon him) sensed the odour of these two from a person in a mosque, he was made to go to al-Baqi’. So he who eats it should (make its odour) die by cooking it well.