Is it true that the phrase "instill the greatness of Allah in your hearts" is an innovation (bid‘ah) because it was not transmitted from the Messenger of Allah, peace and blessings be upon him?

There is no difference between saying: "Bring to mind the greatness of Allah in your hearts" and "in your chests," for the hearts are in the chests. This statement is not an innovation, because exhortation does not necessarily have to be what was reported from the Messenger. Rather, it can be anything that softens hearts and does not contradict the Qur’an and Sunnah (definition). The Salaf indeed exhorted with beautiful words that were not mentioned in the Qur’an or Sunnah, and they were not censured for it.