“The Pharisees,

And all the Jews,

Do not eat

Unless they wash

Their hands.

Thus,

They observe

The tradition

Of the elders.”

οἱ γὰρ Φαρισαῖοι καὶ πάντες οἱ Ἰουδαῖοι ἐὰν μὴ πυγμῇ νίψωνται τὰς χεῖρας οὐκ ἐσθίουσιν, κρατοῦντες τὴν παράδοσιν τῶν πρεσβυτέρων,

There is nothing like this elsewhere, because Mark was explaining this Jewish practice to his gentile Christian readers.  Mark said that the Pharisees (οἱ γὰρ Φαρισαῖοι) and all the Jews (καὶ πάντες οἱ Ἰουδαῖοι) did not eat. unless they had washed their hands (μὴ πυγμῇ νίψωνται τὰς χεῖρας οὐκ ἐσθίουσιν).  Not washing hands was considered to be not upholding or a violation against the Jewish tradition of the elders or priests (κρατοῦντες τὴν παράδοσιν τῶν πρεσβυτέρων).  The importance of this tradition was clearly seen in Matthew, chapter 15:2, in this more Jewish Christian writing.  It is not clear that all Jews followed this tradition, but the Pharisees certainly did.  Originally, this practice of washing hands before eating was what the priestly Levites did in the Temple to practice ritual purity as indicated in Exodus, chapter 30:17-21.  Yahweh had told Moses that there should be a bronze basin with a bronze stand for washing.  Thus, Aaron and his sons should wash their hands and feet when they went into the meeting tent or the altar.  The penalty for not washing your hands and feet was death under this perpetual ordinance.  However, the Pharisaic oral tradition, or the tradition of the elders, had extended this practice to individual and their own homes.