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