Note:
Not only does Bonhoeffer tie silence and solitude together, but he also connects silence to speech. About solitude and community, remember, he said you can’t safely have one without the other; so here he would say ‘Let him who cannot be silent beware of speech. Let him who is not speaking beware of being silent.’

Quote:
“The mark of solitude is silence, as speech is the mark of community. Silence and speech have the same inner correspondence and difference as do solitude and community. One does not exist without the other. Right speech comes out of silence, and right silence comes out of speech.”

Source: Bonhoeffer, Dietrich. Life Together. San Francisco: HarperSanFrancisco, 1954, page 78. (original German pub in 1939)