Abu Dawud Book 6, Hadith Number 2184.

Narated By ‘Aisha, Ummul Mu’minin : The Prophet (PBUH) said: The divorce of a slave-woman consists in saying it twice and her waiting period is two menstrual courses (qur’) AbuAsim said: A similar tradition has been narrated to me by Muzahir and al-Qasim on the authority of ‘Aisha from the Prophet (PBUH), except that he said: And her waiting period (‘iddah) is two courses.