Code:
#include 
#include 
#include 
#include 
#include 
#include 

int read (FILE* source)
{
    char word[1024]="", str[1024];
	if (!fgets(str, 1023, source)) return 0;

    if (str[0]=='1')
	{
    FILE* file = fopen ("1.txt", "a");
    fputs (str, file);
    fclose (file);
	}
	if (str[0]=='2')
	{
    FILE* file = fopen ("2.txt", "a");
    fputs (str, file);
    fclose (file);
	}
    if (str[0]=='3')
	{
    FILE* file = fopen ("3.txt", "a");
    fputs (str, file);
    fclose (file);
	}
	if (str[0]=='4')
	{
    FILE* file = fopen ("4.txt", "a");
    fputs (str, file);
    fclose (file);
	}    
	if (str[0]=='5')
	{
    FILE* file = fopen ("5.txt", "a");
    fputs (str, file);
    fclose (file);
	}    
	if (str[0]=='6')
	{
    FILE* file = fopen ("6.txt", "a");
    fputs (str, file);
    fclose (file);
	}    
	return 1;
}

void main()
{
    FILE* source = fopen ("source.txt", "r");
	while (read (source));
	fclose (source);
}