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)

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

The source seems to be R' Sholom Schwadron

It is by no means concrete evidence but an article on Page 8 in this publication quotes this parable with the following source:

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

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)