Question:

Somewhere in the Bible, doesn't it say, if something is a sin to you, then it's a sin?

Answer:

   
It almost sounds that way, but we should be clear.  Just because I don't think something is a sin, that does not mean that it is not a sin.  God get's to define sin and righteousness.  We do not.  That's because He is God and we are not.  Whatever is not of faith is sin.  Romans 14:23  That means, if God didn't tell you to do it, then it is sin.  The way is very narrow.  We are to be in God's presence at all times hearing His voice.   What this means is that God directs.  Whatever we do that is not by His direction is sin.  For instance, if we pray for things that He has not directed us to pray for, it is sin.  If we start a corporation to minister the gospel and He didn't ask us to do it, then it is sin.  
   
You may be thinking of this Scripture:    
   

1 Corinthians 10:18 Behold Israel after the flesh: are not they which eat of the sacrifices partakers of the altar?  

10:19 What say I then? that the idol is any thing, or that which is offered in sacrifice to idols is any thing?    

10:20 But I say, that the things which the Gentiles sacrifice, they sacrifice to devils, and not to God: and I would not that ye should have fellowship with devils.      

10:21 Ye cannot drink the cup of the Lord, and the cup of devils: ye cannot be partakers of the Lord's table, and of the table of devils.        

10:22 Do we provoke the Lord to jealousy? are we stronger than he?          

10:23 All things are lawful for me, but all things are not expedient: all things are lawful for me, but all things edify not.            

10:24 Let no man seek his own, but every man another's wealth.              

10:25 Whatsoever is sold in the shambles, that eat, asking no question for conscience sake:                

10:26 For the earth is the Lord's, and the fulness thereof.                  

10:27 If any of them that believe not bid you to a feast, and ye be disposed to go; whatsoever is set before you, eat, asking no question for conscience sake.                    

10:28 But if any man say to you, This is offered in sacrifice to idols, eat not for his sake that shewed it, and for conscience sake: for the earth is the Lord's, and the fulness thereof:                      

10:29 Conscience, I say, not thine own, but of the other: for why is my liberty judged of another man's conscience?                        

10:30 For if I by grace be a partaker, why am I evil spoken of for that for which I give thanks?                          

10:31 Whether therefore ye eat, or drink, or whatsoever ye do, do all to the glory of God.                            

10:32 Give none offense, neither to the Jews, nor to the Gentiles, nor to the church of God:                              

10:33 Even as I please all men in all things, not seeking mine own profit, but the profit of many, that they may be saved.