Helaman 6:22, the Book of Mormon: 1
“22 And it came to pass that they did have their signs, yea, their secret signs, and their secret words; and this that they might distinguish a brother who had entered into the covenant, that whatsoever wickedness his brother should do he should not be injured by his brother, nor by those who did belong to his band, who had taken this covenant.”
Excerpt from the LDS Endowment Ceremony (pre 1990): 2
“I will now explain the covenant and obligation of secrecy which are associated with this token, its name, sign, and penalty, which you will be required to take upon yourselves.
Before doing this, however, we desire to impress upon your minds the sacred character of the first token of the Aaronic priesthood, with its accompanying name, sign, and penalty, as well as that of all the other tokens of the holy priesthood, with their names, signs, and penalties, which you will receive in the temple this day. They are most sacred and are guarded by solemn covenants and obligations of secrecy to the effect that under no condition, even at the peril of your life, will you ever divulge them, except at a certain place that will be shown you hereafter.”