Adam Clarke on Psalms 89:19

Then thou spakest in vision to thy holy one - Instead of חסידך chasidecha, "thy holy one," חסידיך chasideycha, "thy holy ones," is the reading of sisty-three of Kennicott's and seventy-one of De Rossi's MSS., and a great number of editions besides.

If we take it in the singular, it most probably means Samuel, and refers to the revelation God gave to him relative to his appointment of David to be king in the stead of Saul. If we take it in the plural, it may mean not only Samuel, but also Nathan and Gad.

For what God revealed to Samuel relative to David, see 2Sam 7:5, etc.; 1Chr 11:2, 1Chr 11:3; and for what he said to Nathan on the same subject, see 1Chr 17:3, 1Chr 17:7-15. All the Versions have the word in the plural.