RDIMMs are implicitly ECC.
There's UDIMMs (typical RAM from desktops), and then RDIMMs (Registered), and LRDIMMs (Load-Reduced DIMM).
Because RDIMMs and LRDIMMs are only used on servers, they're all ECC / Error Correction just implicitly. At least, I've never seen an RDIMM or LRDIMM missing ECC.