While this story appears in many mystical traditions, this is the Sufi version.
A conventionally-minded dervish, from an austerely pious school, was walking one day along a river bank. He was absorbed in concentration upon moralistic and scholastic problems, for this was the form which Sufi teaching had taken in the community to which he belonged. . . .
Suddenly his thoughts were interrupted by a loud shout: someone was repeating the dervish call. ‘There is no point in that,’ he said to himself, ‘because the man is mispronouncing the syllables. Instead of intoning YA HU, he is saying U YA HU.’
Then he realized that he had a duty, as a more careful student, to correct this unfortunate person, who might have had no opportunity of being rightly guided, and was therefore probably only doing his best to attune himself with the idea behind the sounds.
So he hired a boat and made his way to the island in midstream from which the sound appeared to come.
Sitting in a reed hut he found a man, dressed in a dervish robe, moving in time to his own repetition of the initiatory phrase. ‘My friend,’ said the first dervish, ‘you are mispronouncing the phrase. It is incumbent upon me to tell you this, because there is merit for him who gives and him who takes advice. This is the way in which you speak it.’ And he told him.
‘Thank you,’ said the other dervish humbly.
The first dervish entered his boat again, full of satisfaction at having done a good deed. After all, it was said that a man who could repeat the sacred formula correctly could even walk upon the waves: something that he had never seen, but always hoped—for some reason—to be able to achieve.
Now he could hear nothing from the reed hut, but he was sure that his lesson had been well taken.
Then he heard a faltering U YA as the second dervish started to repeat the phrase in the old way. . . .
While the first dervish was thinking about this, reflecting upon the perversity of humanity and its persistence in error, he suddenly saw a strange sight. From the island the other dervish was coming towards him, walking on the surface of the water . . . .
Amazed, he stopped rowing. The second dervish walked up to him and said: ‘Brother, I am sorry to trouble you, but I have to come out to ask you again the standard method of making the repetition you were telling me, because I find it difficult to remember it.”
The Man Who Walked on Water from Idries Shah's Tales of the Dervishes. Page 85.