This simple echo bot replies to every private text message.

It uses the @on_message decorator to register a MessageHandler and applies two filters on it: filters.text and filters.private to make sure it will reply to private text messages only.

from pyrogram import Client, filters

app = Client("my_account")

@app.on_message(filters.text & filters.private)
def echo(client, message):
    message.reply_text(message.text)  # Automatically start() and idle()