Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As _
  System.EventArgs) Handles Button1.Click
        sql = "Select * FROM Zeitbuchungen WHERE Z_Pers_Nr=" & TextBox1.Text
        MsgBox(sql)
        cmd.CommandText = sql             ' <----- dieser Teil hat gefehlt
        conn.Open()
        da.Fill(ds, sql)
        DataGridView1.DataSource = ds
        DataGridView1.DataMember = sql
        conn.Close()