Words and Music: Traditional English
(From MS. Sloan 2593, fol. 74, v⁰)

Compare:
A Babe Is Born, All Of A Maid (Version 1)
A Babe Is Born (All of a May - Version 2)

Source: Thomas Wright, Specimens of old Christmas Carols, Selected from Manuscripts and Printed Books (London: The Percy Society, 1841)
Also found in William Sandys, Christmas-tide, Its History, Festivities and Carols, With Their Music (London: John Russell Smith, 1852), p. 226.

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See notes in F A Q

A babe is born al of a may,
In the savasyoun of us,
To hem we syngyn bothe nyõht and day,
    Veni creator spiritus.

At Bedlem that blyssid pas,
The chyld of blysse born he was,
Hymn to serve, go õeve us gras,
    O lux beata trinitas.

Ther come thre kynges out of the est,
To worchepoe the kyng that is so fre,
With gold and myrre and francincens,
    A solis ortus cardine.

The herdes herdyn an aungele cry,
A merye song then sungyn he,
Qwy arn õe so sore a-gast?
    Jam ortus solis cardine.

The aungele comyn doun with on cry,
A fayr song then syngyn he,
In the worchepe of that chyld,
    Gloria tibi, Domine.