Musnad Ahmad 30:298

Yahya narrated to us, from Ibn ‘Ajlan, Sa‘id ibn Abi Sa‘id narrated to me, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "Three, all of them have a right upon Allah to His aid: the striver (mujahid) in the way of Allah, the one who marries seeking chastity, and the slave (mukatab) who seeks to fulfill his contract of manumission."