"God, grant me the serenity to accept the things I cannot change, Courage to change the things I can, And wisdom to know the difference."

This is the best known form and has been attributed to St Francis of Assisi amongst others. Sometimes with certain quotes, the attribution can be difficult to determine. Some online sources wrongly attribute quotes to an individual based on inadequate research or controversy. What happens then is that someone asks a question and then the wrongly attributed quote is used and this is then copied to other websites.

Wiki states this about the serenity prayer: "Reinhold Niebuhr wrote the prayer for use in a sermon, perhaps as early as 1934...
In 2009, however, Duke researcher Stephen Goranson found a variant attributed to Niebuhr in a 1937 Christian student publication: "Father, give us courage to change what must be altered, serenity to accept what cannot be helped, and the insight to know the one from the other.

The prayer has sometimes been attributed to Friedrich Christoph Oetinger (17021782)."

So we can do further research ourselves, or accept what we believe to be the correct attribution if we trust the source material. I always thought that it was St Francis, so there you are. Former UK Prime Minister Margaret Thatcher is reported to have used this quote. Did she know at that time that it was/is also the mantra of Alcoholics Anonymous?