Whoever calls to guidance
مَنْ دَعَا إِلَى هُدًى كَانَ لَهُ مِنَ الْأَجْرِ مِثْلُ أُجُورِ مَنْ تَبِعَهُ، لَا يَنْقُصُ ذَلِكَ مِنْ أُجُورِهِمْ شَيْئًا
Man da'a ila hudan kana lahu mina-l-ajri mithlu ujuri man tabi'ah, la yanqusu dhalika min ujurihim shay'a.
"Whoever calls others to guidance will have a reward like the rewards of those who follow him, without that taking anything away from their rewards."
Narrated by Abu Hurayrah. Sahih Muslim 2674
A short reflection
Teach someone a du'a and every time they say it, you share the reward. Good deeds can keep multiplying long after you have moved on.