Actually, using the Slider as the binding source is better choice. The Text="{Binding StringFormat='{0:F2}'}" will not work for the binding source. And I can't find any way can do that in xaml.
But you can try to format the label's text in code behind when the label is binding source. Such as:
In the xaml:
<Label x:Name="label" PropertyChanged="label_PropertyChanged"/><Slider Value="{Binding Text,Source={Reference label}}" WidthRequest="300"/>And in the code behind:
private void label_PropertyChanged(object sender, System.ComponentModel.PropertyChangedEventArgs e) { if(e.PropertyName == "Text") { (sender as Label).Text = String.Format("{0:0.##}",Convert.ToDouble((sender as Label).Text)); } }