Halakhah su Genesi 21:30

And I have also heard from the mouth of sages that there is a great secret in the matter of the Jubilee and that all the days and years of the world are hinted to in it (see Ramban on Leviticus 25:2 and Ibn Ezra on Leviticus 25:2). Also in the matter of the sevens - that we were commanded to count the years seven at a time, and not eight at a time or nine at a time, or less than [seven] - did they say that there is a great and goodly matter of wisdom about it as well. They have known it, but did not want to give it over to every person. And even though their secret was not revealed to us, we have paid heed to this thing - that the span of sevens is arranged in many of our commandments: Behold, we continue our work six days, and we rest on the seventh; we work the land six years, and we rest on the seventh; after seven cycles of seven years, we also rest a year - and that is the Jubilee that we have come [to write] about [here]; behold the holiday of Pesach is seven days, and likewise the holiday of Sukkot is seven, and after the seven, we celebrate [Shemini] Atseret; likewise, we count seven seven-year cycles from Pesach to [Shavuot], and after the tally of the seven, we celebrate [Shavuot]; likewise the cutting of a covenant (treaty), which is something done for preservation of a matter, is on the basis of seven, as it is written (Genesis 21:30), "Rather take these seven sheep from my hand"; likewise Bilaam, who was a wise man, made seven altars; so too, some of the sages said that the word, oath (shevuah) - which is translated [into Aramaic] as preservation - is derived from the expression, seven (shevah); and so [too,] many that I have not raised now on the tip of my pen. And you, my precious son, should merit and research and see and add to your knowledge and understand the matters. But I have already finished my work here to arouse your spirit about the question.