John Calvin on Genesis 21:31-32

31. Wherefore he called that place Beer-sheba . Moses has once already called the place by this name, but proleptically. Now, however, he declares when, and for what reason, the name was given; namely, because there both he and Abimelech had sworn; therefore I translate the term ‘the well of swearing.’ Others translate it ‘the well of seven.’ But Moses plainly derives the word from swearing; nor is it of any consequence that the pronunciation slightly varies from grammatical correctness, which in proper names is not very nicely observed. In fact, Moses does not restrict the etymology to the well , but comprises the whole covenant. I do not, however, deny that Moses might allude to the number seven As the word שבע́ means both an oath and the number seven, room is left for this difference of interpretation. Calvin seems, however, to allude to a notion not uncommon among learned men, that as oaths were often made before seven witnesses, which perhaps the seven lambs represented, Abraham might have this number as well as the oath in his mind, when he called the well Beer-sheba. — Ed