Sahih Bukhari Volume 003, Book 027, Hadith Number 015.
Narated By Al-Aswad : That 'Aisha said, "O Allah's Apostle! The people are returning after performing the two Nusuks (i.e. Hajj and 'Umra) but I am returning with one only?" He said, "Wait till you become clean from your menses and then go to At-Tan'im, assume Ihram (and after performing 'Umra) join us at such-and-such a place. But it (i.e. the reward if 'Umra) is according to your expenses or the hardship (which you will undergo while performing it)."