Answered by Shaykh Amjad Rasheed
Translated by Shaykh Hamza Karamali, SunniPath Academy Teacher
If husband and wife differ as to whether intercourse took place, whose statement takes precedence? Is the purificatory bath (ghusl ) then obligatory on them both or only on the one who believes that the penetration occurred?
If the woman believes that the penetration occurred, then it is obligatory on her to take a purificatory bath. As for the husband, we can infer from what our scholars say in the section on the nullifiers of ablution that he presumes himself to remain in his initial state, namely purity [since it was not removed with certainty.] This legal maxim is given precedence over his being informed of the contrary by his wife. 
This applies when the husband firmly believes that she is mistaken or when he doubts whether or not she is correct. If, however, he believes her, then the purificatory bath is obligatory on him, by way of analogy to similar situations where our scholars took this belief into account. This was stated by our teacher, Shaykh Muhammad al-Khatib (may Allah preserve him).
 This is inferred from what Qalyubi says in his marginal glosses on the commentary of Mahalli on the Minhaj (1.37): “If someone divinely protected from error informs him that something has exited from his private parts [causing him to enter into a state of minor ritual impurity], then one must act in accordance with this [information.] If, however, someone who is merely upright [and not divinely protected from error] informs him of this, then one does not act in accordance with what he says — as was the opinion of our teacher — because the presumption of continuity of the initial state is a stronger indication [regarding his state of purity.]
هذا كلُّه فيما لو جزم بخطئها أو شكَّ فيما تقول، أما لو وقع في قلبه صدقُها بحصول الإيلاج فالظاهرُ وجوبُ الغُسل عليه قياساً على مسائل اعتبروا فيها ذلك كما استوجهه شيخنا الخطيب