Maitri Upanishad belongs to the Krishna Yajur Veda. Here are few teachings from Maitri Upanishad.
Borne along and defiled by the stream of qualities¸ unsteady¸ wavering¸ bewildered¸ full of desire¸ distracted¸ one goes on into the state of self-conceit. In thinking¸ ‘This is I’ and ‘That is mine’ one binds himself with himself¸ as does a bird with a snare.
Mind is indeed the source of bondage and also the source of liberation. To be bound to things of this world: this is bondage. To be free from them: this is liberation.
The mind is said to be twofold: the pure and also the impure; impure from contact with desire — pure when freed from!
Even as fire without fuel becomes extinct in its own place, even so thought, by the cessation of activity becomes extinct in its own source.
What a man thinks, that he becomes, this is the eternal mystery.
Maitri Upanishad