Musnad Ahmad 30:2503

Yahya narrated to us, from Ibn ‘Ajlan, who said: Sa‘id narrated to me, from Abu Hurayrah, from the Prophet, peace and blessings of Allah be upon him, who said: "There are three, all of whom it is a right upon Allah, the Almighty and Majestic, to aid: the fighter in the way of Allah, the Almighty and Majestic; the one who marries seeking chastity; and the slave who has a contract of manumission (mukātab) who desires to fulfill the payment."