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.
The first verse that comes to my mind is in Genesis 18:12-


  Therefore Sarah laughed within herself, saying, After I am waxed old shall I have pleasure, my lord being old also?


This is the only verse I can really remember in this context, and it is probably what Peter is referring to.