וּלְקַחְתֶּ֞ם אֲגֻדַּ֣ת אֵז֗וֹב וּטְבַלְתֶּם֮ בַּדָּ֣ם אֲשֶׁר־בַּסַּף֒ וְהִגַּעְתֶּ֤ם אֶל־הַמַּשְׁקוֹף֙ וְאֶל־שְׁתֵּ֣י הַמְּזוּזֹ֔ת מִן־הַדָּ֖ם אֲשֶׁ֣ר בַּסָּ֑ף וְאַתֶּ֗ם לֹ֥א תֵצְא֛וּ אִ֥ישׁ מִפֶּֽתַח־בֵּית֖וֹ עַד־בֹּֽקֶר  

Take a bunch of hyssop, dip it in the blood that is in the basin, and apply some of the blood that is in the basin to the lintel and to the two doorposts. None of you shall go outside the door of your house until morning.

וְעָבַ֣ר יְהֹוָה֮ לִנְגֹּ֣ף אֶת־מִצְרַ֒יִם֒ וְרָאָ֤ה אֶת־הַדָּם֙ עַל־הַמַּשְׁק֔וֹף וְעַ֖ל שְׁתֵּ֣י הַמְּזוּזֹ֑ת וּפָסַ֤ח יְהֹוָה֙ עַל־הַפֶּ֔תַח וְלֹ֤א יִתֵּן֙ הַמַּשְׁחִ֔ית לָבֹ֥א אֶל־בָּתֵּיכֶ֖ם לִנְגֹּֽף 

For יהוה, when going through to smite the Egyptians, will see the blood on the lintel and the two doorposts, and יהוה will pass over the door and not let the Destroyer enter and smite your home. 

Exodus 12:22-23