Musnad Ahmad 962:181

Yahya ibn Ishaq narrated to us, Ibn al-Mubarak narrated to us, Ibn Lahi'ah informed us, from Khalid ibn Abi 'Imran, from the one who narrated to him, from Abu Umamah al-Bahili, who said: I heard the Messenger of Allah (peace and blessings of Allah be upon him) saying: "There are four whose rewards continue for them after death: a man who died while stationed (in ribat) in the way of Allah, and a man who taught knowledge, so its reward continues for him as long as it is acted upon, and a man who set a charity in motion, so its reward continues for him as long as it continues for them, and a man who left behind a righteous child who prays for him."