Look to 2 Thessalonians 2:15 if you have doubts:
St. Paul writes: "Therefore, brothers, stand firm and hold fast to the traditions that you were taught, either by an oral statement or by a letter of ours."
Look also to 2 Timothy 3:14-17:
"But you, remain faithful to what you have learned and believed, because you know from whom you learned it, and that from infancy you have known (the) sacred scriptures, which are capable of giving you wisdom for salvation through faith in Christ Jesus. All scripture is inspired by God and is useful for teaching, for refutation, for correction, and for training in righteousness, so that one who belongs to God may be competent, equipped for every good work" (NAB).
Many Protestants will quote the above verses as if they support sola-scriptura. However, first and foremost, these verses never say that we are saved by the sacred scriptures alone. And most importantly, these verses do not refer to the Bible! When St. Paul wrote this, the Bible had not yet been put together. St. Paul was referring to the Old Testament being useful for teaching. This verse can not be used to support sola-scriptura!
1. Sola scriptura (Scripture alone)
A whole string of powerful objections can be brought against this tenet of Luther and the Protestants; above all, Holy Scripture itself can be called to witness.
a) In John 20.30-1, we read: "Many other signs did Jesus in the presence of the disciples, which are not written in this book." And a little further on (John 21.25): "But there are also many other things which Jesus did; were every one of them to be written, I suppose that the world itself could not contain the books that would be written."
Thus it is clearly stated that Holy Scripture presents only a portion of the world and deeds of Jesus; and it is not at all clear by what criterion the choice was made. It is therefore a wholly unjustified assumption that Holy Scripture alone contains the saving teachings of Christ and that what is left out consists only of unimportant details.
b) Actually the Lord instructed His disciples to go forth and teach. He gave them no commission to write a book. It is therefore the living teaching under the inspiration of the Holy Ghost, the Spirit of Truth, which stands at the beginning of the life of the Church. This fact corresponds to an immediately obvious point: God the Creator of nature as well as of grace makes use of men as His tools, whereby a very special meaning comes to the spoken word in its transmission from person to person.
c) Only in later years was Holy Scripture first written down, after the Church had already existed for many years and had developed her life fully in the celebration of the Holy Sacrifice, the administration of the Sacraments and in the proclamation of the wonderful works of God. If Scripture had been the only definitive foundation of the Church, the Church would not have existed for the first few decades at all.
d) Whose authority is it to decide what belongs to Scripture? Or, to put it another way, what does the canon of Holy Writ contain? The criterion for this cannot be in the Bible itself, as the question can be asked only afterwards. There must therefore be a criterion outside Scripture which discerns and separates the authentically inspired writings from the apocryphal: this is the authority of Christ Himself, which carries a protection of faith under the guidance of the Holy Ghost intact through the storms of the ages.
e) Who interprets in doubtful cases and in disputes as they occur? The Holy Ghost Himself, say Luther and the Protestants. This opinion the Catholic can accept, provided he makes the restriction that the Holy Ghost must be expressed in an objective way by human standards, namely through the Magisterium of the Church, and that the guardianship of the treasury of the faith in every case of doubt be removed from all subjective association. The very number of self-contradictory Protestant sects shows clearly that God has entrusted the treasury of faith neither to private persons, nor to any and all rival groups for final decision.
Here as always, the Protestants have nothing positive with which to replace Catholic teaching. They live entirely and exclusively on criticism of our positions, and they claim that we Catholics are in principle no better than they, the Protestants; that they have Scripture as definitive source of faith, while we on our side have added yet another system of dogmas.
The answer to this argument is simple: the Catholic Church is neither a set of dogmas, nor a system of morality, but is above all, in its sacrifice in our midst, the powerfully living and powerfully working Emmanuel (Christ-with-us). The Church does not have a tradition; rather, the Church essentially is Tradition, more precisely, a prolongation of the Word-made-Flesh. Hence it is not the Church which offers sacrifice, it is not the Church which baptizes, and it is not the Church which teaches, but rather, properly and in the last analysis, it is Christ Who offers sacrifice, baptizes and teaches, and uses human priests and the Pope as Supreme Pontiff as His instruments in the mystery of mediating grace.
The Church is therefore the living Christ, established with a living authority, which is capable at any time of redefining truths (but not of inventing them!) to meet contemporary problems, to discern and sort out, to argue, to judge and to reject. "Whoever hears you, hears Me; whoever rejects you, rejects Me and rejects Him, Who sent Me," said the Lord to His apostles.
The Protestant position, in its one-sided emphasis on the Word, is nothing but cold rationalism. It does not want to acknowledge that the Word became Flesh and sacrificed Himself, that the Redemption is the great work of God in history. It pushes the altar aside and puts in its place the pulpit; the sermon and hymns stand in the center of things, no longer the tenting of God among men. So much the more painful must the reminders of the Reformation within the Church strike the Catholic of our day in light of the above: the rejection of Tradition, the pushing aside of mystery and the march to cold rationalism. As Stuttgart went over to Protestantism in the sixteenth century, the priests celebrated the Holy Sacrifice of the Mass for the last time on a prearranged day in the court chapel. Afterwards the prior took the Blessed Sacrament from the tabernacle; the eternal flame was put out; the building has remained standing to the present day; but He, Emmanuel, is gone.