Ibn 'Abbas (RAA) narrated that Allah's Messenger (ﷺ) said:
"When a man's slave-woman bears him a child she becomes free at his death." Related by Ibn Majah and Al-Hakim with a weak chain of narrators. (ES)
English reference (ES): Book 15, Hadith 1479 (ES)
Arabic reference (ES): Book 15, Hadith 1435 (ES)