Sayyiduna Safinah (may Allah be pleased with him), the freed slave of Sayyidah Umm Salamah (may Allah be pleased with her), says: Sayyidah Umm Salamah (may Allah be pleased with her) freed me and made the condition that I serve the Prophet (peace and blessings be upon him) for the whole of my life.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العتق / 21427