All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
It is not necessary to wash the saliva discharged from the mouth of a sleeping person for it is pure. Imaam Al-Buhooti said: "The semen of a human, the moisture of a woman's vagina, sweat and mucus; phlegm, even if it is bluish, and the saliva of the mouth during sleep are all pure."
This rule applies if the substance is proven to be from the mouth. However, if it is proven to be from the stomach, then it is impure, although many scholars have denied the possibility that it could be from the stomach.
Imaam An-Nawawi said: “I have consulted trustworthy physicians about this issue and they have denied the fact that it could be from the stomach, and they denounced those who deem washing it as obligatory.”
However, he elaborated on this issue aptly:
“The most predominant opinion in this regard is that it is not obligatory to wash, unless it is known to be from the stomach. As long as it is mere doubt, then washing is not obligatory and may be conducted out of precaution. If it is proven impure and one experiences it excessively, then the predominant opinion in this regard is that one is exempted from washing. The same ruling applies to the blood of fleas, urinary incontinence, vaginal bleeding outside the menstrual period and the like which are pardoned due to hardship. And Allaah knows best.”
We should also point out that if the substance is impure, then it should be washed with water, as previously clarified, during wet or dry ablution, if this substance constitutes a layer that prevents water or dust from reaching the skin.
Allaah Knows best.