The midfielder is leaving Athletic this summer after 15 seasons.
Muniain wrote: "A farewell is like a penalty in a Cup final.
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"There can be no doubt.
"You take the word, you place it with care, you secure your footing, six steps back, you move forward and write it with your heart.
"The word is thank you.
"Thank you.
"My dear, my dear Athletic.
"My love, my love Athletic, I love you forever and forever Athletic.
"I will always love you."