One of the adherents of the Alter Rebbe was such a simple chassid that people doubted his knowledge of the meaning of the prayers. Nonetheless he prayed at length, even during the weekday prayers.

The chassidim were amazed that, in spite of his simplicity, he prayed with great depth and fervency. They questioned him about this.

“I only know that which I heard from the [Alter] Rebbe: ‘Zachor v’shamor bedibur echad [The words] “remember” and “guard” [the Shabbos] (were uttered by G‑d) in one breath.’ The Rebbe interpreted this to mean: One must remember and guard in one’s every utterance (breath) the echad, the oneness of G‑d,” he responded.

It was with this Torah thought from the [Alter] Rebbe that he prayed for forty consecutive years.

Sefer HaSichos 5696, p. 127; Likkutei Sichos, vol. 14, p. 224