Riyad Us-Saliheen (Gardens of the Righteous)

Chapter 339
Abomination of Joining As-Salat (The Prayer) when the Food is ready or when one is in desperate need to answer the call of Nature

1753. `Aishah (May Allah be pleased with her) said: I heard the Messenger of Allah (PBUH) saying, "No Salat (prayer) should be performed when the food has been served, nor should it be performed when a person is in need of relieving himself.''
[Muslim].

Commentary:  We learn from this Hadith that one should not offer Salat when he is hungry and the food has been served or has to answer the call of nature. The reason behind is that if one offers Salat in that condition, he will not be able to do it with full concentration. Similarly, if one has to answer the call of nature, he should first do so and then offer Salat.