There is an interesting Pasuk in this week's Parsha, תולדות. Speaking of Yitzchak's tremendous wealth it says, "וימצא בשנה ההיא מאה שערים", that he found that year 100 measures. (The word שער can mean "gate" or "measure" as in שער יציג). The Midrash says that Yitzchak received 100 times more than what was reasonably expected. The Torah bothered to give a measurement in order to tell us that the Patriarchs were careful to set the example of giving tithes or a tenth of their earnings. Avraham gave 10% to מלכי צדק who was referred to as a כהן. (The של״ה הקודש claims that אהרון הכהן was a reincarnation of מלכי צדק). Yaakov made a pledge to Hashem that he would give מעשר, or a tenth of what he received. And Yitzchak carefully measured his harvest to be sure that he gave his מעשר. The message here is to emphasize the importance of giving our own מעשר to צדקה. We see from Yitzchak the abundant blessings that are received because of the fulfillment of this difficult Mitzva.