Midrasch zu Jeschijahu 59:21

The Holy One, blessed be He, made a covenant with the Israelites that neither they nor their descendants, unto the last generation, would forget the Oral Law, as it is said: And as for Me this covenant with them, saith the Lord. My spirit that is upon thee, and My words that I have put in thy mouth, shall not depart out of thy mouth, nor out of the mouth of thy seed, nor out of the mouth of thy seed’s seed (Isa. 59:21). It is not written in this verse “from thee,” but rather out of thy mouth, nor out of the mouth of thy seed, nor out of the mouth of thy seed’s seed.

R. Parnach said in the name of R. Jochanan: "He who is a scholar himself, and also his son and also his grandson are scholars, the Torah will not depart from his children for everlasting; as it is said (Is. 59, 20) And My words which I have put in thy mouth shall not depart out of thy mouth, nor out of the mouth of thy children, nor of the mouth of thy children's children, said the Lord, from henceforth — and unto all eternity." What does the second, "said the Lord," mean? This means that the Holy One, praised be He, said: "I am the surety that so it will continue." What does for henceforth and unto all eternity, mean? R. Jeremiah said: "From now on the Torah returns to its old inn."