You want to do something like this:
if @Mode= 'Searching'
BEGIN
Select [Hadith_Text]
,[Hadith_Urdu]
,[Hadith_English]
,[Chapter_English_Name]
,[Chapter_Urdu_Name]
,[Baab_English_Name]
,[Baab_Urdu_Name]
,[Baab_Id]
,[Hadith_Book_Number]
From Hadiths_old
Where Baab_Id < 100 + ' ' + @WhereClause
order by ID
END
This will work, provided your "@WhereClause" variable is assigned the value of "BookSelection" in your C# code.