Location and date? This looks like a Harrier species (white rump, gray body, flat face/head, long tail, flying low over field), but we need to know where you saw it to be able to tell you which species it is.
Looks like a juvenile Red-shouldered or Broad-winged Hawk. I'm leaning Red-shouldered because of the evenly-spaced, relatively fine streaking on the breast and the less compact body. A photo of its wings would help.
Edit: akandula beat me! Glad we agree. 🙂