Musnad Ahmad 30:3151

'Abd al-Rahman ibn Mahdi narrated to us, he said: Zuhayr—meaning Ibn Muhammad—narrated to us, from al-'Ala', from his father, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "Let not one of you say: 'My slave (male)' and 'My slave (female),' for all of you are slaves of Allah, and all of your women are slave-women of Allah. But let him say: 'My boy' and 'My girl,' and 'My young man' and 'My young woman.'"