Charles Ellicott Commentary Exodus 30:36

"and thou shalt beat some of it very small, and put of it before the testimony in the tent of meeting, where I will meet with thee: it shall be unto you most holy." — Exodus 30:36 (ASV)

You shall ... put it before the testimony. — Some pieces of the incense were to be continually before the ark of the covenant, either on the golden altar, or perhaps at its base ready for offering. This would symbolize the need of the perpetual offering of prayer.