Does swallowing saliva break one's fast?
What should one do with it if they cannot swallow it?
Please answer providing good evidences or references.
No, it doesn't.
Narrated Abu Huraira:
The Prophet said, "If somebody eats or drinks forgetfully then he should complete his fast, for what he has eaten or drunk, has been given to him by Allah." Narrated 'Amir bin Rabi'a, "I saw the Prophet cleaning his teeth with Siwak while he was fasting so many times as I can't count." And narrated Abu Huraira, "The Prophet said, 'But for my fear that it would be hard for my followers, I would have ordered them to clean their teeth with Siwak on every performance of ablution." The same is narrated by Jabir and Zaid bin Khalid from the Prophet who did not differentiate between a fasting and a nonfasting person in this respect (using Siwak).
Aisha said, "The Prophet said, "It (i.e. Siwak) is a purification for the mouth and it is a way of seeking Allah's pleasures." Ata' and Qatada said, "There is no harm in swallowing the resultant saliva."
No it does not. Because saliva is produced within the body itself and is not something that we put in from outside. It does not cause nourishment and is the body's natural process.
Similarly if he sticks out his tongue with saliva on it, then brings it back in and swallows what is on it (of saliva), that does not break his fast according to the more correct view, because however the tongue moves it is regarded as part of the inside of the mouth, the nature of which cannot change, so it does not matter what is on it (of saliva).
Haashiyat Qalyoobi, 2/72.
Same goes for mucus (phlegm).
More info http://islamqa.info/en/ref/49005
Swallowing saliva does not invalidate the fast even if it has been collected in mouth because of thoughts about sour things.
As far as I'm concerned the criteria on which the Islamic rules prohibiting eating and drinking for a fasting person, are based is that whether swallowing something is considered "eating" or "drinking" in the GENERAL ACCEPTED CONCEPT (URF)
I don't think that swallowing saliva is considered "drinking" according to URF.
Though there are many who believe swallowing one's own saliva is forbidden, there are some good references against this tradition. However the prophet used to suck Aisha's tongue during fasting and used to drink her saliva.
Fasting (Kitab Al-Siyam) (800)Chapter: The Fasting Person Swallowing Saliva(34) باب الصَّائِمِ يَبْلَعُ الرِّيقَ Narrated Aisha, Ummul Mu'minin: The Prophet (ﷺ) used to kiss her and suck her tongue when he was fasting.
حَدَّثَنَا مُحَمَّدُ بْنُ عِيسَى، حَدَّثَنَا مُحَمَّدُ بْنُ دِينَارٍ، حَدَّثَنَا سَعْدُ بْنُ أَوْسٍ الْعَبْدِيُّ، عَنْ مِصْدَعٍ أَبِي يَحْيَى، عَنْ عَائِشَةَ، أَنَّ النَّبِيَّ صلى الله عليه وسلم كَانَ يُقَبِّلُهَا وَهُوَ صَائِمٌ وَيَمُصُّ لِسَانَهَا . Grade : Da'if (Al-Albani) ضعيف (الألباني) حكم : Reference : Sunan Abi Dawud 2386 In-book reference : Book 14, Hadith 74 English translation : Book 13, Hadith 2380