First word??

Last night, I was sitting on the floor with my son while he played. He was making lots of noises including the “m” sound.

I said to him, “Can you say “mama”?

He said, “mama”.

I immediately looked at my husband on the couch, and he was looking at me with a face of amazement. I swear our son said it quite clearly, and I teared up. He hasn’t said it again though.

Does this count as his first word? Or was he just copying me? Does he have to say it independently to be considered a first word? Thoughts?