Abu Hurayrah (may Allah be pleased with him) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) said: "If a man invites his wife to his bed and she refuses, and so he spends the night angry with her, the angels will curse her until the morning."
This Hadīth indicates that a woman should answer her husband's request of having sexual intercourse with her, or else she exposes herself to the angels' curse. This is restricted, however, by the husband's anger, as in Al-Bukhāri's narration. If he does not object to her refusal, there is no problem. This also applies if the wife has a legitimate excuse, such as being too sick to have sex or having another excuse that prevents her from coming to the husband's bed. Other than that, she has to respond to him. On the other side, the husband has to fulfill the sexual needs of his wife, if he notices that she wants to have sexual intercourse or asks him to do so. Allah says: {And live with them in kindness.} [Sūrat An-Nisā': 19]