Billy Crystal Saw Rob and Michele Reiner's Slain Bodies

Billy Crystal and his wife Janice saw the slain bodies of Rob Reiner and his wife Michele ... because TMZ has learned they arrived at the scene in a flash after a family member called them about the horrific discovery.

Sources with direct knowledge tell TMZ ... Rob and Michele's daughter Romy called Crystal and his wife not long after she called police to report finding her parents brutally murdered inside their Brentwood home Sunday.

Sources with direct knowledge tell us Billy and his wife raced over to the house -- and were there so quickly, one source tells us -- the couple actually saw their slain friends ... and wanted to say their goodbyes.

As you know, Rob and Michele were found Sunday afternoon … their son Nick was located and arrested on suspicion of murder about six hours later … he's currently being held without bail on suicide watch.