LVII. That one should not fast or prostrate eitrher on Sunday or in Eastertide.
If a presbyter because of the public penance which ge received from the priest (sacerdos), or because of another necessity fasts on Sunday out of piety, like Manichaeans, let him be anathema. Similarly, as an ancient canon, transmitted from the apostles, states, it pleased us that on all Sundays as well as on the days of Easter up to the Pentecost we shall perform our office of prayer not prostrated or humiliated, but with the face directed to the Lord because in these days we celebrate the joy of the resurrection of the Lord.
(trans. M. Szada)