[(Exodus 12:2:) "THIS NEW MOON" (i.e., "THIS MONTH").] Rabbi Samuel bar Abba said: Every new moon that is born from the sixth hour [of the day] and earlier, the eye has the power to see it; from the sixth hour and later, the eye does not have the power to see it. And that new moon, concerning which the Holy One, blessed be He, spoke to Moses, was born from the sixth hour and later, and the eye did not have the power to see it. So the Holy One, blessed be He, showed it to him with His finger, and said to him: "This new moon."
Why God Pointed to the New Moon for Moses with His Finger
Midrash Tanchuma Buber, Bo 10:1
[החדש הזה]. ר' שמואל בר אבא אמר כל חודש שהוא נולד משש שעות ולמטה, יש כח בעין לראות, משש שעות ולמעלה, אין כח בעין לראותו, ואותו החודש שאמר הקב"ה למש נולד משש שעות ולמעלה, ולא היה כח בעין לראותו, והראהו לו הקב"ה באצבע, ואמר לו החדש הזה.