Sunan Ibn Majah, Volume 3, Book 19, Hadith 2526 (DarusSalam) ⓘ
It was narrated from that Safinah - Abu 'Abdur-Rahman-said : “Umm Salamah freed me but stipulated that I should serve the Prophet as long he lived.”
حَدَّثَنَا عَبْدُ اللَّهِ بْنُ مُعَاوِيَةَ الْجُمَحِيُّ، حَدَّثَنَا حَمَّادُ بْنُ سَلَمَةَ، عَنْ سَعِيدِ بْنِ جُمْهَانَ، عَنْ سَفِينَةَ أَبِي عَبْدِ الرَّحْمَنِ، قَالَ أَعْتَقَتْنِي أُمُّ سَلَمَةَ وَاشْتَرَطَتْ عَلَىَّ أَنْ أَخْدُمَ النَّبِيَّ صلى الله عليه وسلم مَا عَاشَ .
also referenced under: Hadith 2526 (Reference) & Book 19, Hadith 2622 (In-Book)