<TextBox ToolTip="{Binding RelativeSource={x:Static RelativeSource.Self}, Path=(Validation.Errors)[0].ErrorContent}">
    <TextBox.Text>
        <Binding Path="ScriptFileMap" Mode="TwoWay" UpdateSourceTrigger="PropertyChanged">
            <Binding.ValidationRules>
                <v:MinimumStringLengthRule MinimumLength="1" ErrorMessage="Map is required for saving." />
            </Binding.ValidationRules>
        </Binding>
    </TextBox.Text>
</TextBox>

public void SaveScript()
{
    if (this.ScriptFileName.Length > 0 && this.ScriptFileMap.Length > 0)
    {
        // save function logic
    }
}

<TextBox Text="{Binding Path=ScriptFileName, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" BorderBrush="{Binding Path=ScriptFileNameBorder, UpdateSourceTrigger=PropertyChanged}" ToolTip="{Binding Path=ScriptFileNameToolTip, UpdateSourceTrigger=PropertyChanged}" />

public string ScriptFileName
        {
            get
            {
                return this.scriptFileName;
            }

            set
            {
                this.scriptFileName = value;
                RaisePropertyChanged(() => ScriptFileName);

                if (this.ScriptFileName.Length > 0)
                {
                    this.ScriptFileNameBorder = borderBrushNormal;
                    this.scriptFileNameToolTip.Content = "Enter the name of the file.";
                }
                else
                {
                    this.ScriptFileNameBorder = Brushes.Red;
                    this.scriptFileNameToolTip.Content = "File name is required for saving.";
                }
            }
        }