Bereshit 37:21 בראשית ל״ז:כ״א
1 א

לא נכנו נפש. קשה היה לראובן עצת האחים והיתה כוונתו להצילו, ואילו אמר לא נכנו סתם, היה מראה עצמו בדבריו כי מניעתו זאת לחמלה עליו, וע"כ הוסיף להזכיר נפש כלומר שאינו מקפיד עליו רק עליהם, שלא יכו נפש שום אדם. וכן מה שאמר אל תשפכו דם ולא אמר דמו, כלומר גם אנכי ככם שנאתיו ואין אני מקפיד על דמו כלל כי אם עליכם שלא תהיו שופכי דם.

. לא נכנו נפש, “let us not commit murder. Reuven found it impossible to agree with the advice, i.e. the solution proposed by his brothers. He intended to save Joseph. Had he said immediately: “we will not kill him,” he would have given away his intention to save Joseph and the fact that he had pity on him. He therefore added the word נפש to convince his brothers that his strategy was not aimed at saving Joseph’s fate but at saving his brothers from committing an unpardonable sin The prohibition of murdering someone in cold blood is absolute and knows of no exceptions. This is why he added אל תשפכו דם, “do not spill blood.” Had he been concerned only with Joseph’s blood not being spilled he would have said אל תשפכו דמו, “do not spill his blood.” He implied that he shared the brothers’ evaluation of Joseph’s guilt and hated him just as they hated him, but that he was concerned with the basic prohibition of shedding the blood of a human being, נפש.