Canonical version
When ‘Abdu’l-Bahá was in Stuttgart, Germany, in 1913, He related an incident…
When ‘Abdu’l-Bahá was in Stuttgart, Germany, in 1913, He related an incident from His early childhood: ‘It is good to be a spreader of the Teachings of God in childhood. I was a teacher in this Cause at the age of this child (eight or nine years). This reminds me of a story. There was a man, highly educated, but not a Bahá’í. I, but a child, was to make of him a believer. The brother of this man brought him to me. I stayed with him, to teach him. He said, “I am not convinced, I am not satisfied.” I answered, “If water were offered to a thirsty one, he would drink and be satisfied. He would take the glass. But you are not thirsty. Were you thirsty, then you too would be satisfied. A man with seeing eyes sees. I can speak of the sun to every seeing one, and say it is a sign of the day; but a blind person would not be convinced because he cannot see the sun. If I say to a man with good hearing, listen to the beautiful music, he would then listen and be made happy thereby. But if you play the most beautiful music in the presence of a deaf man, he would hear nothing. Now go and receive seeing eyes and hearing ears, then I will speak further with you on this subject.” He went; but later he returned. Then he understood and became a good Bahá’í. This happened when I was very young.’
Source: Honnold, Annamarie, Vignettes from the Life of ‘Abdu’l-Bahá, p. 118
Collected from bahaistories.com (Subject: teaching).