The true content of prayer is not expressed in what is said, whence, among other things, the great mistake of analyzing prayer on the basis of the apparent content of the discourse, and the distinction between the prayer of petition, of praise, of intercession, etc. That sort of thing can be useful from the pedagogical point of view, but it falsifies the true nature of prayer.
Prayer is not a discourse. It is a form of life, the life with God. That is why it is not confined to the moment of verbal statement. The latter (verbalization) can only be the secondary expression of the relationship with God, an overflow from the encounter between the living God and the living person.
Prayer is not to be analyzed like a language. It has none of that form or content, for it receives its content, not from what I have to say, but from the One to whom it is spoken. For prayer to be what it is meant to be, it depends on Him and not on me, still less on my ability to speak the adequate language. Of course, I can pronounce a discourse supposedly addressed to God. I can arrange the sentences, but it is neither the harmony of the form, nor the elevation of the content, nor the fullness of the information which turns it into a prayer. Insofar as it remains a discourse, it is in fact subject to the language analysis with which we are familiar, but that is always as discourse, that is to say, as “nonprayer.”
It becomes prayer by the decision of God to whom it is addressed. A transformation takes place whereby it is a prayer of Christ or a prayer of the Holy Spirit. That is how we should understand the famous statement of Paul, in which he says that in the last analysis we do not know what the content of our prayer should be (Romans 8:26,27), but that the Holy Spirit himself “intercedes with sighs too deep for words.” This phrase has too often been interpreted as though the Holy Spirit added a little something to our prayer. That is quite incorrect. It is the entire prayer which is the prayer of the Holy Spirit. Only when the Holy Spirit intercedes, and in a way which cannot be expressed, that is, which transcends all verbalizing, all language, then is the prayer prayer, and it is a relationship with God. Prayer is a gift from God, and its reality depends upon Him alone.
From: Prayer and Modern Man by Jacques Ellul
 Ro 8:26.27 Likewise the Spirit also helps in our weaknesses. For we do not know what we should pray for as we ought, but the Spirit Himself makes intercession for us with groanings which cannot be uttered.
27 Now He who searches the hearts knows what the mind of the Spirit is, because He makes intercession for the saints according to the will of God.