Looking on the internet deeply has found these results:

he was playing the piano. is the most popular phrase on the web.

he was playing with the piano 11 results on the web	he was playing the piano. 13,300 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
Apr 1, 2013 ... The sound is� Anyway, I don't know who he was playing with, the piano player� I can't give a name. A European, French, American, Italian�	It was Gould's mother who taught him to sing along with the music when he was playing the piano at home. By the time he noticed that he had retained the habit�... And then he went nuts again when he heard he was playing the piano alongside Jon and Curtis.� Wess is featured on flute on Quincy Jones' �Jessica's Day,��... but he was playing the piano for�... He's led a strange life; he's a bright guy � a musician, a scholar with a Ph.D in Shakespearian studies and a published poet [M] [T] Mary plays the piano. [M] [T] Can you play the piano? [M] [T] He played piano by ear. [M] [T] He was playing the piano. [M] [T] She plays the piano by ear.

he was playing with the piano

11 results on the web

he was playing the piano.

13,300 results on the web

More popular!

Apr 1, 2013 ... The sound is� Anyway, I don't know who he was playing with, the piano player� I can't give a name. A European, French, American, Italian�

It was Gould's mother who taught him to sing along with the music when he was playing the piano at home. By the time he noticed that he had retained the habit�...
And then he went nuts again when he heard he was playing the piano alongside Jon and Curtis.� Wess is featured on flute on Quincy Jones' �Jessica's Day,��...
but he was playing the piano for�...
He's led a strange life; he's a bright guy � a musician, a scholar with a Ph.D in Shakespearian studies and a published poet
[M] [T] Mary plays the piano. [M] [T] Can you play the piano? [M] [T] He played piano by ear. [M] [T] He was playing the piano. [M] [T] She plays the piano by ear.