Ruling on rinsing the mouth whilst fasting

The believer is enjoined to do wudoo’ properly. This was the
command of the Prophet (peace and blessings of Allaah be upon him) who
said: “Do wudoo’ properly, make water reach between the fingers and toes,
and rinse the nose very thoroughly, except when you are fasting.” (Narrated
by al-Tirmidhi, al-Sawm, 788; Abu Dawood, 142; classed as saheeh by
al-Albaani in Saheeh Sunan al-Tirmidhi, 631). 

So he (peace and
blessings of Allaah be upon him) pointed out that we should avoid excessive
rinsing of the mouth and nose when fasting, lest that lead to the thing
which is forbidden, namely water getting into the throat whilst fasting. As
for simply rinsing out the mouth whilst fasting, there is nothing wrong with
that so long as the water does not enter the throat of the fasting person. 

Hence it was narrated in a saheeh hadeeth that ‘Umar ibn
al-Khattaab said: “I kissed (my wife) whilst I was fasting. I said, ‘O
Messenger of Allaah, today I did something grievous, I kissed whilst I was
fasting.’ He said, ‘What do you think about rinsing your mouth whilst you
are fasting?’ I said, ‘There is nothing wrong with it.’ He said, ‘Then
what’s wrong?.’”

(Narrated by Abu Dawood, al-Sawm, 2037; classed as
saheeh by al-Albaani in Saheeh Sunan Abi Dawood, no. 2089). 

The commentator on this hadeeth said
concerning the words, “What do you think about rinsing your mouth whilst you
are fasting?”, This is a sign of brilliant understanding, for rinsing the
mouth does not invalidate the fast and it is the first step towards
drinking… And Allaah knows best.