Musnad Ahmad 30:1723

Sulayman ibn Dawud narrated to us, Isma'il informed us, al-'Ala' informed me, from his father, from Abu Hurayrah, that the Prophet, peace and blessings of Allah be upon him, said: "When a human dies, his work is cut off from him, except for three: except for an ongoing charity, or knowledge that is benefited from, or a righteous child who supplicates for him."