app/views/sessions/_form.html.erb
<%= form_for :session, :url => session_path, html: { class: 'form-horizontal' } do |form| %>
<fieldset>
<div class="form-group">
<%= form.label :email, class: 'col-sm-3 control-label' %>
<div class="col-sm-9">
<%= form.email_field :email, class: 'form-control' %>
</div>
</div>
<div class="form-group">
<%= form.label :password, class: 'col-sm-3 control-label' %>
<div class="col-sm-9">
<%= form.password_field :password, class: 'form-control' %>
</div>
</div>
</fieldset>
<div class="form-actions col-sm-offset-3 col-sm-9">
<%= form.submit class: 'btn btn-primary' %>
<%= link_to t('.forgot_password'), new_password_path %>
</div>
<% end %>