The source seems to be R' Sholom Schwadron

Rabbi Ephraim Schwartz says he heard this parable from R' Schwadron himself. In [this blog post][1] he writes:

> There’s a great story I remember from when I was in Yeshiva, back in
> the day a million years ago, when the great Maggid, Reb Shalom
> Shwadron came to speak to us. He told a story of how he was once
> walking through a beautiful field on a gorgeous day. The sun was
> shining so bright, he was surrounded by flowers and trees, he could
> even hear the birds chirping all around him. Suddenly though he felt
> someone punch him in the chest. He looked around and no one was there.
> Hmmm that seemed strange. A second later though he felt another punch
> in his chest. This time though he heard himself mumble words.
> 
> Selach lanu Hashem ki chatanu, mechal lanu ki fasashanu- forgive me
> Hashem for we have sinned. And then it hit him- excuse the pun- he had
> hit himself for he was in middle of Shemona Esrei and just started
> daydreaming.

The parable is also quoted in an article on Page 8 of [this publication][2] with the following source:

> (Heard from Rabbi Moshe Grossman in the name of Rabbi Sholom Schwadron
> zt”l)

[This Hebrew website][3] also attributes the parable to R' Schwadron providing a specific location of this market/fair.

> הגאון רבי שלום שבדרון זצ"ל נהג לספר על יהודי שהסתובב בשוק מחנה יהודה
> כאשר הוא תר אחר דוכן בו מוכרים תפוזים... ופתאום - בום! הוא קיבל אגרוף
> בחזה... ולאחר כמה שניות שוב פעם אגרוף בחזה! כאשר הוא ניסה לבדוק מי מכה
> אותו באמצע השוק הוא תפס כי הוא עצמו מכה על חזהו - בברכת סלח לנו

  [1]: https://holylandinsights.blogspot.com/2024/09/guilt-free-responsibility-parshat.html
  [2]: https://s3.amazonaws.com/s3.flipdocs.com/books/10002264/648715/648715.pdf
  [3]: https://shofar.tv/articles/15011