Or HaChaim on Genesis 12:12/אור החיים על בראשית י״ב:י״ב

1 והיה כי יראו אותך המצרים, It will be that as soon as the Egyptians will set eyes on you, etc. Abraham explained the reason he was going to engage in deception before he asked Sarah to lie. He wanted Sarah to speak about her brother Abraham not merely in response to questions about their status. He wanted her to make it plain even before they entered the land of Egypt that they were travelling together as brother and sister. In the event that Sarah would object to telling lies when none had as yet been called for, he told her that if they merely awaited developments it would be too late. The Egyptians would automatically assume that Abraham was Sarah's husband and they would get rid of him. They would not even bother to ask about her status.