Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates: The Prophet (peace and blessings be upon him) instructed us to show kindness, in freeing slaves, to children born of zina (that is, to free them).
Imam al-Tabarani recorded this narration in al-Mu'jam al-Awsat; its chain contains a narrator, Zakariyya bin Yahya al-Madini, whom I do not know; the remaining narrators of this narration are thiqat (trustworthy).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب العتق / 7276