(Cod. Sang.) 390, fol. 54 (detail).
"In praesepio jacebat et in cælis fulgebat; ad nos veniebat, et apud Patrem manebat."
In a manger he was lying and in the heavens he was shining; to us he was coming, and with the Father he was remaining.
Antiphon to the Benedictus, Morning Prayer, Weekday no. 1 (Monday) between 2 January and Epiphany. According to CANTUS, this antiphon dates back to the late 10th century at least. (But I haven't yet checked any sources beyond Hesbert's Antiphonale missarum sextuplex, in which it does not appear.)
"shining" is too tame for "fulgebat" (flashing, burning, gleaming, shimmering). Unfortunately, nothing better comes to mind. "Fulgurating" doesn't really speak today.
In any case, there is a clear if passing echo here of Sermo 200 by St. Augustine, his second on the Epiphany (PL 38, col. 1029 (1028-1031)), preached on 6 January between 393 and 405 inclusive, and read in the old Roman Breviary "in the second nocturn of the second and third days within the octave of Epiphany" (ACW 15, trans. Thomas Comerford Lawler, 159n1 on 226):
Here we have a great mystery. He was at that time lying in a manger, and yet was leading the Magi from the East. He lay hidden in a stable, yet He was acknowledged in the heavens, so that He, acknowledged in the heavens, might be made manifest in the stable, and this day might be called 'Epiphany,' which may be expressed in Latin as 'Manifestatio.'
[WSA translation here.]
Magnum sacramentum. In præsepi tunc jacebat, et Magos ab Oriente ducebat. Abscondebatur in stabulo, et agnoscebatur in cœlo; ut agnitus in cœlo manifestaretur in stabulo, et appellaretur Epiphania dies iste, quod latine manifestation dici potest.
Thus, "in the heavens he was shining" is probably a reference to the star that led "the Magi from the East", though there are no occurrences of fulg* in Mt 1 (but cf. Mt 24:27 (with Lk 17:24), 28:3; Lk 11:36, 24:4; 2 Cor 4:4; etc.).