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