It was narrated from that Safinah - Abu 'Abdur-Rahman-said :
"Umm Salamah freed me but stipulated that I should serve the Prophet (ﷺ) as long he lived." (ID)
English reference (ID): Vol. 3, Book 19, Hadith 2526 (ID)
Arabic reference (ID): Book 19, Hadith 2622 (ID)