Islam, as we know, lets a husband divorce & then retain a woman three times, after that, man can't remarry his wife until another man marries & divorces her!
Well, Prophet Muhammad explain it thus:
The new husband must "taste her sweetness!" as the previous husband had, before she can return to her previous husband.
All from Sahih Muslim:
Book 008, Number 3357:
'A'isha (Allah be pleased with her) reported that Allah's Messenger (may peace be upon him) was asked about a woman whom a man married and then divorced her, and then she married (another) person, and she was divorced before sexual intercourse with her, whether it was lawful for her first husband (to marry her in this state).
He (the Holy Prophet) said: No, until he has tasted her sweetness. --------------------------------------------------------------------------------
Book 008, Number 3358:
A hadith like this has been narrated on the authority of Hisham with the same chain of transmitters.
--------------------------------------------------------------------------------
Book 008, Number 3359:
'A'Asha (Allah be pleased with her) reported: A person divorced his wife by three pronouncements; then another person married her and he also divorced her without having sexual intercourse with her. Then the first husband of her intended to remarry her. It was about such a case that Allah's Messenger (may peace be upon him) was asked, whereupon he said:
No, until the second one has tasted her sweetness as the first one had tasted. Does anything apart from sex & porn like statements enter the Prophet's mind? If he was alive today, he'd be a porn star!