Mishkat al-Masabih 10:92

Hajjaj b. Hajjaj al-Aslami quoted his father as saying, “Messenger of God, what will remove from me the obligation due for fostering a child.”* He replied, “A slave or a slave-girl of good quality.” *It is said that Arabs liked to give a woman who had fostered a child something over and above the hire. This tradition deals with the amount to be considered full compensation for services rendered. Tirmidhi, Abu Dawud, Nasa’i and Darimi transmitted it.