How can I make sure that after the bot writes the text, my message is deleted?

Question

Python

The bot outputs text on its behalf to the chat and deletes my message

import discord
from discord.ext import commands

TOKEN = 'token'

bot = commands.Bot(command_prefix='!')

@bot.command(pass_context=True)
async def say (ctx, *arg):
await ctx.send(' '.join(arg))
bot.run(TOKEN)