It is not obvious how Hagar connects with the covenant from Mount Sinai. This is the only time in Scripture that such a statement is made.1 However, given the fact that Sarah is connected with the Abrahamic covenant (Isaiah 51:1–2), one can understand how further reflection would result in Hagar being connected with Moses.
24 Now this may be interpreted allegorically: these women are two covenants. One is from Mount Sinai, bearing children for slavery; she is Hagar.