Negative commandment · Lo Ta’aseh #287 of 613
For a relative not to act as a witness
שלא יעיד קרוב
The verse · Devarim 24:16
לֹא־יוּמְתוּ אָבוֹת עַל־בָּנִים וּבָנִים לֹא־יוּמְתוּ עַל־אָבוֹת אִישׁ בְּחֶטְאוֹ יוּמָתוּ
“Fathers shall not be put to death for children, neither shall children be put to death for fathers: every man shall be put to death for his own sin.”
The Rambam’s words
For a relative not to act as a witness, as [Deuteronomy 24:16] states: "Fathers shall not die because of children." According to the oral tradition, we have learned [that the verse teaches that] fathers should not die through the testimony of [their] sons. The same law applies regarding other relatives.
שלא יעיד קרוב, שנאמר "לֹא יוּמְתוּ אָבוֹת עַל בָּנִים" (דברים כד, טז); מפי השמועה למדו שלא יומתו אבות בעדות בנים, והוא הדין לשאר הקרובים.
Why · Sefer HaChinuch #589
That a relative not testify, one about the other
“It is from the roots of this commandment [that] since the essence of all human matters is dependent upon testimony of people — like the matter that I wrote at the beginning of my book — therefore, the Omnipresent wanted to distance [us from] making [improper] judgment among people; [and] only [to make it] with strong and true testimony that is clean from any suspicion.”
Who & when
“And this commandment is practiced, with regards to financial cases that are observed today, in every place and at all times by males, as it is upon them to receive testimony — since they are the ones that judge, and not females.”
Where in the Mishneh Torah
Hilchot Testimony הלכות עדות