Hadith: It is a Sunnah for a man who marries a virgin when he already has a wife (who is no longer a virgin) that he stay seven nights with the new wife and then divide his overnight stay between both. If he marries a previously-married woman when he already has a wife (either a virgin or not), the Sunnah is for him to stay with her (the new wife) three nights and then divide his overnight stay between both

عن أنس رضي الله عنه قال: «من السُّنَّة إذا تزوَّج الرجل البِكْرَ على الثَّيِّب أقام عندها سبْعا وقَسَم، وإذا تزوَّج الثَّيِّب على البِكْر أقام عندها ثلاثا ثم قَسَم» قال أبو قِلابة: ولو شئتُ لقلتُ: إنَّ أنَسًا رَفَعَه إلى النبي صلى الله عليه وسلم .
[صحيح] - [متفق عليه]

Anas (may Allah be pleased with him) was reported to have said: "It is a Sunnah for a man who marries a virgin when he already has a wife (who is no longer a virgin) that he stay seven nights with the new wife and then divide his overnight stay between both. If he marries a previously-married woman when he already has a wife (either a virgin or not), the Sunnah is for him to stay with her (the new wife) three nights and then divide his overnight stay between both." Abu Qilābah said: "If I wanted to do so, I could say that Anas attributed (this Hadīth) to the Prophet (may Allah's peace and blessings be upon him)."
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

This Hadīth highlights an established act of the Sunnah regarding a man who marries a new wife when he already has one or more wives: If the new wife is a virgin, he should stay seven nights with her and then divide his time among his wives, including the new one. If the new wife is a non-virgin, the husband should stay three nights with her and then divide his time among all. The discrimination between the virgin and the non-virgin is because the virgin needs more time to feel less alienated and shy, unlike a previously-married woman, who needs this less than a virgin. Also, a man usually has a stronger desire for a virgin, that is why the Islamic law gives him the chance to satisfy his desire and enjoy the pleasure of marriage by staying with his virgin wife for this period of time.