Sahih Bukhari Volume 004, Book 055, Hadith Number 578.
Narated By Abu Huraira : Abraham did not tell a lie except on three occasion. 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 alright. 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)."