A poem by an unknown author on the craft of masonry uses "so mote it be."
"An admonishment about Mass and how to behave in Church (excerpt), Poem on the craft of masonry (The Regius Manuscript)," Royal MS 17 A I. ca. 1400–1450, in Digitised Manuscripts, The British Library
Middle English Transcription:
Grante me ye blysse · withoute ende; amen amen · so mot hyt be ·
Now, swete lady · pray for me.
This thou myght say, or su[m] other thynge,
When thou knelust · At the sakerynge
For covetyse aft[er] good, spare thou nought
to worshipe hym · all' hath wroght.
Modern English Translation:
Grant me the bliss without end;
Amen! Amen! so mote it be!
Now, sweet lady, pray for me
Thus thou might say, or some other thing,
When thou kneel at the sacrament
For covetousness after good, spare thou not
To worship him all hath wrought.