Person asking: …A man took into his mouth his wife’s saliva during the daytime in Ramadan, and his saliva mixed with her saliva, and he swallowed [that]. So has he broken his fast, and what’s the [Islāmic] ruling, with evidence?
The Shaikh: Naturally, if he had swallowed or tasted the sweetness of her saliva, he has broken his fast since he ingested something from outside his own body, so he is like one who has eaten or drunk [something].
Person asking: Meaning by this he [must] make up this day [of fasting]?
The Shaikh: No. If he had done that knowing [that it breaks the fast], he is [considered] one breaking the fast intentionally. Meaning, as for if he [had] done that without realizing, that does him no harm.
Person asking: And what here is said to be the ruling along with evidence?
The Shaikh: The evidence . . . if someone were to ask me if one who takes a bit of salt and swallows it breaks his fast and [then asks me] what’s the evidence?! The evidence is that he ingested something of food or drink; then, if it were sugar, what’s your opinion? [It’s] sweeter and sweeter. And if it were his wife’s saliva? [It’s] sweeter and sweeter and sweeter!
Person asking: May Allāh grant you a good reward.
N.B.: Title mine (Trans.)