“Appreciation is the highest form of prayer, for it acknowledges the presence of good wherever you shine the light of your thankful thoughts.”
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Related quotes

“If the only prayer you say throughout your life is "Thank You," then that will be enough.”

“If "thank you" is the only prayer you can utter in your lifetime, that would be enough.”
Very commonly attributed to Eckhart on the internet and some publications, the source of the first formulation however is: A Bucket of Surprises (2002) by J. John and Mark Stibbe.
Disputed

“Thanks cows. I appreciate your tastiness.”

“If you only utter a single prayer in this life let it be Thanks, with all your heart”
All Will be Well (2004)

Source: What I Know For Sure