The angel announces the birth to the virgin not after the conception, lest she should be thereby too much troubled, but before the conception he addresses her, not in a dream, but standing by her in visible shape. For as great indeed were the tidings she receives, she needed before the issue of the event an extraordinary visible manifestation.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: sup. Mat. Hom. 4.