Turnus Rufus asked this question also of Rabbi Akiva, "Why is the
Sabbath distinguished from other days?" Rabbi Akiva replied, "Why art
thou distinguished from other men?" The answer was, "Because it hath
pleased my Master thus to honor me." And so retorted Akiva, "It hath
pleased God to honor His Sabbath." "But what I mean," replied the other,
"was how dost thou know that it is the Sabbath-day?" The reply was, "The
river Sambatyon proves it; the necromancer proves it; the grave of thy
father proves it, for the smoke thereof rises not on the Sabbath."
THE TALMUD, _Sanhedrin_, fol. 65, col. 2.