What is the ruling on the fasting person swallowing saliva?
Saliva does not harm the fast because it is only spit. If the person swallows it, there is no harm in that and if he spits it out, there is no harm.
In reference to phlegm, which is that which comes up from the chest or the nose and it is also called mucus, and it is thick phlegm which the person sometimes gets from the chest and sometimes from the head, the man and woman must spit it out. It is obligatory to expel it and refrain from swallowing it (for the fasting person). Concerning normal saliva, which is spit there is no problem with it and it does not harm the fasting person, whether a man or a woman.
Shaykh `Abdul-`Azeez Bin Baz
Fatawa Islamiyah Page no.260 Vol: 2
Darussalam English publication