Musannaf Ibn Abi Shayba 7:397

Waki’ narrated to us, from Thawr al-Shami, from ‘Abd al-Rahman ibn Sa’d, who said: A man came to Ibn ‘Abbas and said: "I have two slave boys; one of them is legitimate (rashdah) and the other is illegitimate (ghayyah), and I wish to emancipate one of them. Which of the two do you see that I should emancipate?" He said: "The one of them that is of greater value, even if he is a child of adultery."