
Lord Meher http://www.lordmeher.org/index.jsp?pageBase=page.jsp&nextPage=6742, by Bhau Kalchuri, pp. 5770, 5970, 6742. The famous Bobby McFerrin song was based on this phrase often used by Meher Baba.
Variants:
Don't Worry; be Happy! Remember Me; I will help you.
As quoted in Showers of Grace (1984) by Bal Natu, p. 92
Do your best, then don't worry, be happy. Leave the rest to me.
As quoted in The Homeopathic Revolution : Why Famous People and Cultural Heroes Choose Homeopathy (2007) by Dana Ullman, p. 367.
General sources
Variant: Don't worry, be happy.