In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
It is certainly acceptable to swallow your own saliva; in no way does it break your fast.
Responding to your question, Sheikh Ahmad Kutty, a senior lecturer and an Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states:
There is certainly nothing wrong with swallowing your own saliva. If such a thing had been considered as invalidating the fast, then the Prophet (peace be upon him) would have said so clearly. For it is a vital piece of information that everyone needs to know. We believe that the Prophet (peace be upon him) communicated to the Ummah in no uncertain terms everything that we must understand, especially regarding matters of worship.
Allah says in the Quran: “Eat and drink until the white thread of dawn becomes distinct from the black thread of dawn (until the appearance of true dawn).” (Al-Baqarah 2:187)
So, we are told to abstain from food and drink while fasting. As Imam Ibn Hazm rightly pointed out, human beings do not reckon things such as swallowing one’s own saliva, rinsing the mouth, or making istinja (cleaning oneself with water after urination or defecation) as akin to eating and drinking.
In conclusion, it is certainly acceptable to swallow one’s own saliva; in no way does it break one’s fast.
Almighty Allah knows best.