When did Sarah call Abraham "lord"?

Peter says that Sarah called Abraham "lord" (i.e., master) and that it was a good thing. When did she do this? And how does Peter know this? I can't find it anywhere in scripture. > **1 Peter 3:5,6 ESV** For this is how the holy women who hoped in God used to adorn themselves, by submitting to their own husbands, **as Sarah obeyed Abraham, calling him lord**. And you are her children, if you do good and do not fear anything that is frightening.