Reference: Fataawa Ramadhaan – Volume 2, Page 540, Fatwa No.517
Fataawa Ibn ‘Uthaymeen – Volume 1, Page 515
Question: Does the swallowing of phlegm break the fast?
Response: According to the madh.hab [of Imaam Ahmad], if the phlegm does not reach the mouth, then it does not break the fast. If, however, it does reach the mouth, then spit it out.
[Regarding the issue of when it reaches the mouth] then there are two opinions amongst the people of knowledge:
From amongst them are those who say: Indeed it does break the fast because it is analogous to food and drink.
And from amongst them are those who say: It does not break the fast because it is analogous to saliva, since saliva does not break the fast even of he gathered up saliva [in his mouth] and swallowed it, then his fast would not be invalidated.
And this is the preponderant opinion because there is no evidence as to the invalidity of his fast.