Hadith: Safīnah said: I was Umm Salāmah’s slave, and she said: I will free you on condition that you serve the Messenger of Allah (may Allah's peace and blessings be upon him) for as long as you live

عن سفينة، قال: كنت مَمْلُوكًا لأم سلمة فقالت: أُعْتِقُكَ وأشترط عليك أن تخدم رسول الله صلى الله عليه وسلم ما عِشْتَ فقلت: «وإن لم تَشْتَرِطِي عَلَيَّ، ما فَارَقْتُ رسول الله صلى الله عليه وسلم ما عِشْتُ فَأَعْتَقَتْنِي، واشْتَرَطَتْ عَلَيَّ».
[حسن] - [رواه أبو داود وابن ماجه وأحمد]

Safīnah (may Allah be pleased with him) reported: I was Umm Salāmah’s slave, and she said: "I will free you on condition that you serve the Messenger of Allah (may Allah's peace and blessings be upon him) for as long as you live." I replied: "Even if you do not make this a condition, I will never desert the Messenger of Allah (may Allah's peace and blessings be upon him) for as long as I live." She manumitted me pursuant to this condition.
Hasan/Sound. - [Ibn Maajah]

Explanation

In this Hadīth, Safīnah (may Allah be pleased with him) relates that he was a slave of Umm Salāmah (may Allah be pleased with her) and she freed him on condition that he served the Messenger of Allah (may Allah’s peace and blessings be upon him) for the rest of his life. He told her that, even if she had not stipulated this condition, he would not desert the Prophet (may Allah's peace and blessings be upon him) as long as he lived. She manumitted him subject to this condition. The Hadīth indicates that it is valid to stipulate a condition in relation to the manumission of slaves.