Srila Prabhupada's Letter to Janardana, April 26, 1968:
On the whole, you may know that he is not a liberated person, and therefore, he cannot initiate any person to Krishna Consciousness. It requires special spiritual benediction from higher authorities.
Srila Prabhupada's Letter to Satsvarupa, September 20, 1968:
"Our relationship is eternal. But if somebody lags behind, so in spite of our eternal relationship one may not meet other at the destination. Just like a flock of birds--although very intimately related, everyone of them has to fly in the sky by individual strength. If one is less strong, the other cannot keep him in the sky. That is the law of nature. So long everyone of us is strong in Krishna Consciousness, there is no doubt, you can fly in the spiritual sky and meet together without failure."
Srila Prabhupada's Letter to Jayapataka, July 11, 1969:
And anyone who defies and therefore disconnects the relationship with the Spiritual Master can hardly expect the assistance of the Spiritual Master life after life.
Srimad-Bhagavatam 1.19.37 Purport - The Appearance of Sukadeva Gosvami:
The pseudo spiritual master flatters the so-called disciple, and thereby both the master and his ward go to hell without a doubt.
Srila Prabhupada's Letter to Tusta Krsna, December 14, 1972:
As for your next question, can only a few pure devotees deliver others, anyone, if he is a pure devotee he can deliver others, he can become spiritual master. But unless he on that platform he should not attempt it. Then both of them will to go to hell, like blind men leading the blind.
Srimad-Bhagavatam 6.7.14 - Indra Offends His Spiritual Master, Brhaspati:
"Leaders who have fallen into ignorance and who mislead people by directing them to the path of destruction [as described in the previous verse] are, in effect, boarding a stone boat, and so too are those who blindly follow them. A stone boat would be unable to float and would sink in the water with its passengers. Similarly, those who mislead people go to hell, and their followers go with them."
Caitanya-caritamrta Madhya 15.108 -- The Lord Accepts Prasada at the House of Sarvabhauma Bhattacarya:
"Unless one is initiated by a bona fide spiritual master, all his devotional activities are useless. A person who is not properly initiated can descend again into the animal species."