Floyd Mayweather ... Witnessed Murder-Suicide on FaceTime

Floyd Mayweather is reeling tonight ... because he was on FaceTime with one of his best friends when the guy pulled out a gun, shot his wife and then killed himself ... TMZ has learned.

Sources tell us ... rapper Earl Hayes called Floyd on FaceTime Monday morning in a rage, over claims his wife -- VH1 star Stephanie Moseley -- had been unfaithful.

According to our Floyd sources, Earl said he was going to kill his wife. The champ was pleading with him to get a grip ... to no avail. Floyd will not say how much he saw, but he acknowledges he was a witness and heard everything.