אַל־תִּתֵּן֙ אֶת־אֲמָ֣תְךָ֔ לִפְנֵ֖י בַּת־בְּלִיָּ֑עַל כִּֽי־מֵרֹ֥ב שִׂיחִ֛י וְכַעְסִ֖י דִּבַּ֥רְתִּי עַד־הֵֽנָּה׃
«求你不要讓你的婢女在那卑劣的女人面前蒙羞。我一直說到現在，是因為我心裡有太多苦情和愁煩。»
Siftei Chakhamim
Do not say this (לא תאמר כן)... Rashi added in the wordsתאמר כן (say this) because Yoseif had declared them as spies, so the proper word [to deny this] would be אַל. So we see with Channah (Shmuel I, 1:16). Eli thought she was a drunk, and she replied [in denial]: אַל תתן אמתך לפני בת בליעל. Whereas לא [when standing alone] is the proper reply to a question or a request.