The source seems to be R' Sholom Schwadron
Rabbi Ephraim Schwartz says he heard this parable from R' Schwadron himself. In this blog post 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 with the following source:
(Heard from Rabbi Moshe Grossman in the name of Rabbi Sholom Schwadron
zt”l)
This Hebrew website 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
