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." (PT)
English reference (PT): Vol. 3, Book 19, Hadith 2526 (PT)
Arabic reference (PT): Book 19, Hadith 2622 (PT)