Answered by Shaykh Yūsuf Badāt
Is it permissible to swallow saliva while fasting? Does it break the fast?
Jazāk Allāh Khayr/ Thank you for your question.
Swallowing one’s own saliva will not invalidate the fast. (See: Badā’i al Ṣanā’i)
The act of one deliberately gathering, collecting and swallowing one’s own saliva, to quench thirst, while fasting, is disliked – makrūh – (See: Marāqī al Falāḥ)
And Allāh Knows Best