Silsilat al-Ahadith al-Sahihah

Narrated from Sayyiduna Sulaiman (may Allah be pleased with him) that I heard the Messenger of Allah (peace and blessings be upon him) say: "The reward for four deeds of the living continues to reach the dead as well (these four deeds are): The deceased has such successors who pray for him, their prayer reaches him; the deceased leaves behind ongoing charity, as long as its benefit continues among the living, he keeps receiving its reward; the deceased taught such knowledge which is acted upon after him, he receives a reward equal to that of the one acting upon it, and the doer's reward is not diminished in any way; and the deceased who died while guarding the frontier, he will continue to receive the reward for this deed until the Day of Resurrection."