Albert Barnes Commentary Psalms 99:7

"He spake unto them in the pillar of cloud: They kept his testimonies, And the statute that he gave them." — Psalms 99:7 (ASV)

He spake unto them in the cloudy pillar—He spoke to the men of other times, to those who called upon His name.

It cannot be meant literally that He spoke to "Samuel" from the "cloudy pillar"—the pillar that guided the Israelites in the wilderness—unless that term is understood in the general sense as denoting the "Shechinah," the visible symbol of the divine presence, the cloud that rested on the ark.

The idea is that God spoke to His people in ancient times from the cloud, the symbol of His presence. He communed with them; He heard their prayers; He gave them His commandments; He interposed in their behalf; and it was not a vain thing that they worshipped Him.

All this was as true of Samuel—it is as true now of those who call upon God—as it was of Moses and Aaron.

They kept his testimonies ...—They obeyed His laws, and He thus heard and answered them.