Brochas Daf Yud Daled amud alpeh

Rav said that anyone who greets his fellow man prior to davening (Shemone Esre) is regarded as if he made that person into a Bameh (an illicit altar)

The Maharshah comments that tefillah is “b’makom avodas korban” and thus one who greets his friend with the salutation, Shalom (which is a name of HKBH) prior to praying, looks as if he has gone to sacrifice elsewhere.

The Ben Yehoyada takes a somewhat different approach – at the end of the Amidah the concluding blessing is “sim Shalom” which itself concludes with the words, “es amo Yisrael b’Shalom.”  Thus a person greeting his friend “Shalom” appears as if beseeching HKBH is less important to him than seeking his Yeshuas from his fellow man.