Well, it would be impossible to know who originaly thought such a thought. A concept such as "accept this moment as it is" probably goes all the way back to whenever humans first began contemplating such things.

Does one need to credit every piece of advice to someone else who may have given that same advice before? For example, if I write an article about how to be a better person, and one of my pieces of advice is that you should clean your room, do I need to credit that bit of advice to someone else? For example, psychologist Jordan Peterson has said it, but how many millions of other people have said if before him?