Ofcourse, I think by reading the whole hadith its fairly obvious:
Narrated Abu Huraira:
The Prophet said, "The Prophet Abraham emigrated with Sarah and entered a village where there was a king or a tyrant. (The king) was told that Abraham had entered (the village) accompanied by a woman who was one of the most charming women. So, the king sent for Abraham and asked, 'O Abraham! Who is this lady accompanying you?' Abraham replied, 'She is my sister (i.e. in religion).' Then Abraham returned to her and said, 'Do not contradict my statement, for I have informed them that you are my sister. By Allah, there are no true believers on this land except you and I.' Then Abraham sent her to the king. When the king got to her, she got up and performed ablution, prayed and said, 'O Allah! If I have believed in You and Your Apostle, and have saved my private parts from everybody except my husband, then please do not let this pagan overpower me.' On that the king fell in a mood of agitation and started moving his legs. Seeing the condition of the king, Sarah said, 'O Allah! If he should die, the people will say that I have killed him.' The king regained his power, and proceeded towards her but she got up again and performed ablution, prayed and said, 'O Allah! If I have believed in You and Your Apostle and have kept my private parts safe from all except my husband, then please do not let this pagan overpower me.' The king again fell in a mood of agitation and started moving his legs. On seeing that state of the king, Sarah said, 'O Allah! If he should die, the people will say that I have killed him.' The king got either two or three attacks, and after recovering from the last attack he said, 'By Allah! You have sent a satan to me. Take her to Abraham and give her Ajar.' So she came back to Abraham and said, 'Allah humiliated the pagan and gave us a slavegirl for service." (Sahih al-Bukhari, Volume 3, Book 34, Number 420)
here's another hadith regarding this;
Narrated Abu Huraira:
Abraham did not tell a lie except on three occasions. Twice for the Sake of Allah when he said, "I am sick," and he said, "(I have not done this but) the big idol has done it." The (third was) that while Abraham and Sarah (his wife) were going (on a journey) they passed by (the territory of) a tyrant. Someone said to the tyrant, "This man (i.e. Abraham) is accompanied by a very charming lady." So, he sent for Abraham and asked him about Sarah saying, "Who is this lady?" Abraham said, "She is my sister." Abraham went to Sarah and said, "O Sarah! There are no believers on the surface of the earth except you and I. This man asked me about you and I have told him that you are my sister, so don't contradict my statement." The tyrant then called Sarah and when she went to him, he tried to take hold of her with his hand, but (his hand got stiff and) he was confounded. He asked Sarah. "Pray to Allah for me, and I shall not harm you." So Sarah asked Allah to cure him and he got cured. He tried to take hold of her for the second time, but (his hand got as stiff as or stiffer than before and) was more confounded. He again requested Sarah, "Pray to Allah for me, and I will not harm you." Sarah asked Allah again and he became all right. He then called one of his guards (who had brought her) and said, "You have not brought me a human being but have brought me a devil." The tyrant then gave Hajar as a girl-servant to Sarah. Sarah came back (to Abraham) while he was praying. Abraham, gesturing with his hand, asked, "What has happened?" She replied, "Allah has spoiled the evil plot of the infidel (or immoral person) and gave me Hajar for service." (Abu Huraira then addressed his listeners saying, "That (Hajar) was your mother, O Bani Ma-is-Sama (i.e. the Arabs, the descendants of Ishmael, Hajar's son)." (Sahih al-Bukhari, Volume 4, Book 55, Number 578)