From Sahih Muslim (Hadith 7538):
A'isha said in connection with these words of Allah, the Exalted and Glorious:" And if a woman has reason to fear ill-treatment from her husband or that he might turn away from her" that it was revealed in case of a woman who lived with a person and perhaps he does not want to prolong (his relationship with her) whereas she has had sexual relationship with him (and as a result thereof) she got a child from him and she does not like that she should be divorced, so she says to him: I permit you to live with the other wife
Reference: Muslim 7538