Narrated Ibn 'Umar (RA):

The divorce of a slave-woman occurs by saying it (the divorce word) twice, and her 'Iddah (waiting period) is two menses. [Reported by ad-Daraqutni].

وَعَنِ اِبْنِ عُمَرَ رَضِيَ اَللَّهُ عَنْهُمَا قَالَ: { طَلَاقُ اَلْأَمَةِ تَطْلِيقَتَانِ, وَعِدَّتُهَا حَيْضَتَانِ } رَوَاهُ اَلدَّارَقُطْنِيُّ [1]‏ .‏