If he [Muhammad] divorce you, [Allah] will give him in your stead wives better than you, submissive (to Allah), believing, pious, penitent, devout, inclined to fasting, widows and maids. 66:5
O Prophet! Why bannest thou that which Allah hath made lawful for thee, seeking to please thy wives?"
Allah says it's OK for Muhammad to have sex with any of his wives whenever he wants. 66:1