I am looking for the first extant record of Sunday being referred to specifically as the "Sabbath" or "Christian Sabbath"

I am not asking which day the early church gathered.

I am not asking which day is the Christian Sabbath

I am asking for the first record of Sunday being labeled the Christian equivalent of the Jewish Sabbath.

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Context of my question:

It is quite evident to me that in many early church writings that the idea of the Jewish Sabbath is actually rejected. Sunday was certainly a common day of gathering and to the Jewish believers the seventh day would still have possibly been thought of as the Sabbath day. But from Justin Martyr through the Council of Laodicea a Judaizing celebration of any Sabbath is discouraged and the true Sabbath is not any singular day, but as a status in Christ and an attitude of repentance and rest in Him.

I understand many may even disagree with the above statements, but I tell you this so you can understand what kind of statement I am looking for and the question can be answered regardless. At some point in Christian theology someone labeled Sunday not just a day to gather or even a day to rest and enjoy **a** Sabbath, but someone labeled it as **the** Christian Sabbath. A true shift of the same Sabbath from the Jewish seventh day to a Christian first day.

Finding this early record would certainly impact answers to related questions, but all that is outside the bounds of this question.

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When is Sunday first recorded as being called **the** Sabbath? Not just a day of rest or gathering or worship.