What tree is this? 03-03-20

This is a volunteer tree growing in my yard.  I don't know what it is and was surprised to see it bearing fruit this year.  It isn't a fig or a lemon, although the fruit is that shape.  The birds devoured the fruit, which tells why this was a volunteer tree, planted by birds, no doubt.  

A little larger than a golf ball. 

#   #   #