I teach the Junior class at SS. In the teacher's helps there is a similar story. Perhaps this is the one you are referring to? Here goes:
"..in connection with not being weary in well-doing, tell of the amazing experience of Stephen Grellet.
"He once promised to preach to some lumbermen in the backwoods of America. When he arrived, he found only empty shanties, for the cutters had gone into the forest. However, when the hour arrived for him to preach, he felt God wanted him to do what he had promised. He stood and gave the message, hoping that someone would come back to the camp in time to hear at least part of it; however, no one appeared.
"He left discouraged, considering his time wasted. Many years later, as he was walking across the London Bridge, a man stopped him and said, "I have found you at at last!"
"Pardon me, but I think you are mistaken, for I do not believe I know you," said Stephan Grellet.
"That is true, but I know you, sir," said the first gentleman. "Did you not speak on a certain day at a certain place in the backwoods of America?"
"Yes", said Grellet, "but no one came to listen."
"Well, I was there! I had to return to the encampment to get some tools. I saw you through he trees and stood there and listened. When you finished preaching, I was convicted of my sin an received Christ. Since then I have been telling men the blessed news of salvation. Over one thousand have professed Christ and three of them are missionaries!"
"Stephan wept. He realized as never before that obeying God was not in vain!"
Hope this story will bless you like it did me.