It is narrated from Ibn Abbas and Ibn Zubair (may Allah be pleased with them both) that they said: "A woman who has obtained khula should not be divorced during her waiting period, because now the husband has divorced her and she is no longer his wife."
Hadith Referenceمسند الإمام الشافعي / كتاب عشرة النساء والإيلاء والخلع / 1237