Safinah (RAA) narrated, 'I was a slave of Umm Salamah and she said, 'I shall emancipate you, but on the condition that you serve Allah's Messenger (ﷺ) as long as you live.' Related by Ahmad, Abu Dawud, An-Nasa'i and Al-Hakim. (IT)
English reference (IT): Book 15, Hadith 1471 (IT)
Arabic reference (IT): Book 15, Hadith 1427 (IT)