Sahih Muslim Book 7, Hadith Number 2759.

Chapter : Permissibility of entering into the state Ihram conditionally due to illness.

Ibn Abbas reported that Duba’a bint al-Zubair b. ‘Abd al-Muttalib (Allah be pleased with her) came to Allah’s Messenger (may peace be upon him) and said: I am an ailing woman but I intend to perform Hajj; what you command me (to do)? He (the Holy Prophet) said: Enter into the state of Ihram (uttering these words) of condition: I would be free from it when Thou wouldst detain me. He (the narrator) said: But she was able to complete (the Hajj without breaking down).