Safinah (may Allah be pleased with him) says that Umm Salamah (may Allah be pleased with her) said: I will free you, but I have one condition: that you serve the Messenger of Allah (peace and blessings be upon him) for as long as you live. I said: Even if you did not make this condition, I would not part from the Messenger of Allah (peace and blessings be upon him) for as long as I live. But she made the condition that I serve the Messenger of Allah (peace and blessings be upon him) until my death.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العتق / 21426