Ata (may Allah have mercy on him) narrates from Abdullah bin Abbas and Ibn al-Zubayr (may Allah be pleased with them both) that both of them said, concerning a woman who has taken khul' and whose husband then divorces her: Divorce does not apply to her, because he has divorced what he does not own.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الخلع والطلاق / 14866