Sayyiduna Safinah (may Allah be pleased with him) said: I was a slave of Sayyidah Umm Salamah (may Allah be pleased with her). She said: I will free you, on the condition that you serve the Messenger of Allah (peace and blessings be upon him) as long as you live. I said: Even if you had not set this condition, I would not have parted from the Messenger of Allah (peace and blessings be upon him) as long as I lived. So she freed me and set this condition upon me.
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 6417