Once, while standing on a riverbank next to his timber business near the city of Riga, Reb Binyomin Kletzker (a chassid of the Alter Rebbe) was found deeply immersed in thought, apparently in a deep concept of Chassidus that required profound concentration. He stood in the same position for five consecutive hours. When he came back to himself, he was shocked at the bewildered expressions on the faces of several onlookers. He exclaimed: My wonder and bewilderment [towards you] is greater than yours [toward me], for to be thinking about timber in Riga while reciting the Shema is considered acceptable! But when one meditates on Echad (the unity of G-d) during business... that elicits bewilderment!!

Sefer HaSichos 5686, p. 100, and fns. there