Narrated Umm Salamah, Ummul Mu'minin: Safinah said: I was a slave of Umm Salamah, and she said: I shall emancipate you, but I stipulate that you must serve the Messenger of Allah (ﷺ) as long as you live. I said: Even if you do not make a stipulation, I shall not leave the Messenger of Allah (ﷺ). She then emancipated me and made the stipulation with me.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا مُسَدَّدُ بْنُ مُسَرْهَدٍ، حَدَّثَنَا عَبْدُ الْوَارِثِ، عَنْ سَعِيدِ بْنِ جُمْهَانَ، عَنْ سَفِينَةَ، قَالَ كُنْتُ مَمْلُوكًا لأُمِّ سَلَمَةَ فَقَالَتْ أُعْتِقُكَ وَأَشْتَرِطُ عَلَيْكَ أَنْ تَخْدُمَ رَسُولَ اللَّهِ صلى الله عليه وسلم مَا عِشْتَ . فَقُلْتُ إِنْ لَمْ تَشْتَرِطِي عَلَىَّ مَا فَارَقْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم مَا عِشْتُ فَأَعْتَقَتْنِي وَاشْتَرَطَتْ عَلَىَّ .
sunnah.com