History of the Sabbath and First Day of the Week - John N. Andrews

It should be observed, however, that the earliest
mention of Sunday as the Lord’s Day, is in the
writings of Tertullian; Justin Martyr, some sixty
years before, styling it "the day called Sunday;"
while the authoritative application of that term to
Sunday was by Sylvester, bishop of Rome, more
than one hundred years after the time of Tertullian.
The earliest mention of Sunday as Christian
Sabbath is thus noted by Heylyn:--
"The first whoever used it to denote the Lord’s
Day (the first that I have met with in all this search)
is one Petrus Alfonsus -- he lived about the time
that Rupertus did -- [which was the beginning of
the twelfth century] who calls the Lord’s Day by
the name of Christian Sabbath."[5]
Of Sunday labor in the eastern church, Heylyn
says:--
"It was near nine hundred years from our
Saviour's birth if not quite so much, before restraint
of husbandry on this day had been first thought of
in the east; and probably being thus restrained did
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