וְנֶ֣פֶשׁ כִּֽי־תֶחֱטָ֗א וְשָֽׁמְעָה֙ ק֣וֹל אָלָ֔ה וְה֣וּא עֵ֔ד א֥וֹ רָאָ֖ה א֣וֹ יָדָ֑ע אִם־ל֥וֹא יַגִּ֖יד וְנָשָׂ֥א עֲוֺנֽוֹ׃
누구든지 증인이 되어 맹세시키는 소리를 듣고도 그 본 일이나 아는 일을 진술치 아니하면 죄가 있나니 그 허물이 그에게로 돌아갈 것이요
Akeidat Yitzchak
The ninth commandment addresses itself to the spoken word. Just as the fourth commandment concerns itself with testimony concerning the truth of the Creator and His works, so the ninth commandment concerns itself with testimony concerning events witnessed between man and man.
Nineteen Letters
Your mission, however, is not limited by the walls of your house; beyond their limits you must assist with every particle of your strength wherever it is necessary to save the life, the property, or the happiness of a human being, to assist the enterprise of a fellow man with your strength or fortune, or to help suffering creatures of the lower order wherever you can, by the use of your wealth, your physical or intellectual strength, or your word, support the needy, clothe the naked, feed the hungry, console the mourning, heal the sick, care for the unprovided, advise those in need of counsel, teach the ignorant, reconcile those sundered by anger and quarrel — in a word, to be a blessing whenever and wherever you can.
Sefer Yesodei HaTorah
1) (Vayikra 5:1) "if he hears the voice of an oath (i.e., if he were besworn to give testimony if he knows it), and he were a witness, having seen or known and he does not tell." For one who hears his neighbor beswear him to testify for him if he has testimony on his behalf rationalizes, saying: Who can require me to get involved in a dispute that is not mine and to testify for him? And why should I cause injury to his disputant?