There are various types of Gurus broadly classified into two categories -- Siksha Guru and Diksha Guru. In spiritual matters the later is important.
For the Diksha to be valid, the Diksha Guru has to belong to any definite Parampara (lineage) that starts from God. The mantra was first spoken by God to his immediate disciples. These disciples then pass the mantra to their disciples and gradually in this manner, through an unbroken lineage, the mantra is supposed to reach us.
The mantra (or knowledge) is potent only when we receive it in an unadulterated form exactly how it emerged from the mouth of God. And, this is possible only if the mantra comes down to us through an unbroken lineage of Gurus.
The following verse is the type of verse you are looking for:
vinA dikshAm na mokshah syataduktam shiva-shAsane |
sA cha na
Without Diksha there is no liberation---- this is the order of Shiva;
And, Diksha is not possible without Acharyas (or Gurus); Hence
ParamaparA-s of acharayas have been laid down.
Kularnava Tantram 14.3
This verse says in order to make Diksha fruitful the Paramparas have been laid down. In other words, it says that the Guru (who's giving Diksha) has to be associated with one such Parampara or the other.