source: donald trump rnc GIF by Election 2016 - Find & Share on GIPHY

believe you me or believe me

used to emphasize the truth of a statement or assertion:

｟話｠ 〖挿入的に〗本当だってば, 絶対だよ (!自分を信じるように命令的に強く求める言い方)

e.g. believe me, she is a shrewd woman.