The meaning is either,
1. That God revealed this to Samuel, \see 1 Chronicles 9:22\ or Gad, or Nathan, or some other man of God, who put it into writing, and by them to David. Or,
2. That God did, as it were, by his own hand and finger, (wherewith he wrote the ten commandments, Exodus 31:18) write these things upon the table of my mind, which now I deliver to thee.