Ayesha reported,
A man divorced his wife three times, and she then married another man who divorced her. The Prophet ﷺ was asked whether she could now be lawful for her first husband. He replied, "No, not until he tastes her sweetness as the first one did."
Azami 6577
Sahihعن عائشة زوج النبي ﷺ
أن رجلا طلق امرأته ثلاثا، فتزوجت فطلق، فسئل النبي ﷺ أن تحل للأول؟ قال: لا حتى يذوق عسيلتها كما ذاق الأول.
Narrated by Bukhari in Talaq (5261), and by Muslim in Nikah (115:1433), both through Ubaydullah bin Umar, from Qasim bin Muhammad, from Ayesha, who related it as mentioned.The general wording of Allah's statement, "then if he divorces her, she is not lawful for him afterward until she marries another husband" [Baqarah 2:230], permitted 
the first husband to marry her again after another husband had married her. The sunnah explained that she does not become lawful for the first husband until there has been intercourse between her and the second husband, involving the tasting of the sweetness of intimacy, and she is then separated from him, whether by divorce or death. Only then does she become lawful for the first husband.