Mujahid narrates from Sayyiduna Abdullah bin Abbas (may Allah be pleased with him) concerning the saying of Allah Almighty: "O Prophet, why do you forbid what Allah has made lawful for you?" [Surah al-Tahrim: 1], that he said: He (peace and blessings be upon him) had declared his slave-woman forbidden to himself.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الخلع والطلاق / 15074