By the grace of wisdom and purity of mind, He can be seen, indivisible, in the silence of contemplation. This invisible atman can be seen by the mind wherein the five senses are resting.

Atharva Veda 
Silence is the first door to spiritual eminence.

Adi Sankara
  Things that are real are given and received in silence. God has been everlastingly working in silence, unobserved, unheard, except by those who experience His infinite silence.

Meher Baba
Silence is a source of great strength.

Lao Tzu 
In the school of the Spirit man learns wisdom through humility, knowledge by forgetting, how to speak by silence, how to live by dying.

Johannes Tauler
I have learned silence from the talkative, toleration from the intolerant, and kindness from the unkind.

Khalil Gibran
Silence is one of the great arts of conversation.

Cicero