I don’t know if or when Gandhiji said about happiness or harmony .
But certainly this is a very ancient wise quote from Sanskrit “manasA, vAchA, karmanA” ie in thought, speech and deed.
Manasa, vacha, karmana – Wikipedia
Note : The original Sanskrit terms are almost untranslatable as English words as each word has a long back story and dharmic context. I’ve given very loose translations in English.
This is called sankalpa and defines the 3 forms of Shakti ichha(will), kriya(action), jnana(knowledge) (literally that energy which makes doing anything possible in this universe).
Source : Devi – Wikipedia
In Sita Upanishad, a shakta Upanishad, Sita is extolled as the supreme goddess. The Upanishad identifies Sita with Prakrti (nature) which is constituted by “will” ichha, activity (kriya) and knowledge (jnana).
So what the quote should say is that “Anything becomes possible when one takes sankalpa holding it firmly in one’s thoughts, speech and deed”.
Happiness may or may not follow. But certainly it’ll make things possible to happen.
Obviously when your thoughts, words and actions are in harmony you should be one while rather than split into external and internal faces.
-Thoughts and speech match but actions don’t = paralysis
-Speech and actions match but thoughts don’t = pleasing others
-Thoughts and actions match but Speech doesn’t = hiding real self