John Gill Commentary 1 Chronicles 22:13

"Then shalt thou prosper, if thou observe to do the statutes and the ordinances which Jehovah charged Moses with concerning Israel: be strong, and of good courage; fear not, neither be dismayed." — 1 Chronicles 22:13 (ASV)

Then shall you prosper, if you take heed to fulfil the
statutes
See (1 Kings 2:2 1 Kings 2:3) where the same things are said as here: which shows that this was spoken by David a little before his death.