Praise be to Allaah.
It is not permissible
for a person to start praying when he is resisting the urge to urinate
or defecate, because the Prophet
(peace and blessings of Allaah be upon him) said: “You should not pray
when food is ready or when resisting the urge to urinate or defecate.”
(Narrated by Muslim in his Saheeh).
The reason for that – and Allaah knows best – is that this prevents
a person from having the necessary presence of mind and humility in
the prayer. However, if a person prays in this state, his prayer is
valid although it is somewhat lacking and is imperfect because of the
hadeeth quoted; he does not have to repeat it. But if you start
to pray when you are not resisting the urge to urinate or defecate,
then you start to feel that need during the prayer, then the prayer
is valid and there is nothing makrooh in it, so long as that need does
not prevent you from completing the prayer.
And
Allaah is the source of strength. May Allaah bless our Prophet Muhammad
and his family and companions, and grant them peace.