As a side-note worth to mention I think:
You cannot say "In der Morgen" (would also be fundamentally incorrect since "Morgen " is masculine) or (as long as you solely refer to the daytime morning) "im Morgen" (aka "in dem Morgen"), but you can say the quite similar but more colloquial "In der Früh" (feminine, regionally depended). Worth to mention the grammatical inconsistency here in terms of "an"/"am" vs. "in"/"im". In terms of language consistency and precision (of the context of speech), worth to mention that "in der Früh" is actually the better one here.
This aspect came a bit late into my mind as I read Hubert's answer, so as an update point here:
There is a case, where you can say "in den Morgen": For temporal direction semantics: For instance
"Diese Gesprächsrunde geht bis in den frühen Morgen hinein" for
"This debate lasts until the early morning".
PS: Side-note number two and a bit off-topic but mabye interesting:
You can actually say "Im Morgen" in german within a totally different context, namely with the usage of "Morgen" as "tomorrow" (neuter) while referring to some kind of future foresight for instance:
"Mental lebt er noch im Gestern, Sie schon im Morgen".
"Mentally, he lives in the past, she in the future already."