Majma al-Zawaid — Hadith 768

Sayyiduna Abu Umamah (may Allah be pleased with him) said: I heard the Prophet (peace and blessings be upon him) say: "There are four people whose rewards continue after death: a man who dies while guarding the frontier in the way of Allah; a man who teaches some knowledge, so that as long as that knowledge is acted upon, his reward continues; a man who gives an ongoing charity, so that as long as that charity continues, its reward continues for him; and a man who leaves behind a righteous child who prays for him."
Ahmad recorded it, and al-Tabarani in al-Mu'jam al-Kabir and al-Mu'jam al-Awsat. Al-Bazzar also recorded it. Its chain contains Ibn Lahi'ah and a narrator who is not named.