A friend of mine said that we are not to use instruments when we Worship God in song. If this is true, where in the Bible is this teaching?
You will not find any direct passages prohibiting the use of musical instruments when singing in the assembly of the church. The usual argument (the argument from silence) is that nowhere in the New Testament is there an example of the use of musical instruments other than the human voice by the first century church. Of all the New Testament passages about music in the church, every one mentions singing, but not one mentions playing an instrument. This argument says that since the commands to sing are specific, they rule out any addition, such as mechanical instruments. While this is the argument I was raised with, I consider it one of the weaker arguments.
Slightly stronger is the argument from history. The worship in the assemblies of the early church was patterned on the worship in the Jewish synagogues, which did not allow instruments because that had been part of the Temple worship. When the synagogues developed after the destruction of the First Temple, they forbade the use of instruments. The first attempt at the introduction of instruments into the worship of the church did not occur for several hundred years after Christ. Those attempts met with much resistance, and it wasn't until sometime between 500 and 700 AD that the introduction of instruments into the worship was successful. So from the example of the apostles and early churches we find no authority for the use of instruments; in fact, we find example of forbidding them.
The strongest argument I know, though, is the argument from the shadow and the substance. The most common justification for the use of instruments other than the human voice is that they were allowed in the Temple worship. The things, though of the Tabernacle/Temple were "patterns of things in the heavens." (Heb 9: 23) They were the shadow, of which the things in the church are the substance. Thus, the incense used in the Temple was a shadow of our prayers (Rev 8:3-4). The blood of bulls and goats was a shadow of the sacrifice of Jesus (Heb 9:13-14). In like manner, the instruments in the Temple were a shadow of the ultimate instrument of worship, the human voice. Just as it is unnecessary for Jesus to be offered repeatedly, and just as most do not see the need to burn incense as they pray, so we see there is no need of the shadow of mechanical instruments when we have the substance of the human voice. Anyone who insists that we should or must use instruments in worship because they were used in the Old Testament must necessarily insist on the use of incense while we pray, washing our hands and feet before entering the building, and using oil lamps for lighting inside the building. Is it wrong to use instruments? I can't say for sure. Is it unnecessarily going back to the shadow? Definitely. Can an instrument "speak" or "make melody in the heart" (Eph 5:19), or "teach and admonish" (Col 3:16) like the human voice? Certainly not. Paul warned the Christians in Galatia against going back to the things of the Old Testament for their hope and worship (Gal 3:24-27), saying those who did so have made Christ of no effect and are "fallen from grace." (Gal 5:4)
I hope I have answered your excellent question. Please, in all things spiritual, continue to ask where to find it in the Bible. For that you are to be commended.