Yogananda stated that it is very difficult to reach liberation which is understandable. But he also stated that, "If you practice 1/100th of what I have taught you, you will reach God! In what reference did he make this statement?<
You ask an important question and the answer points out essential lessons for the devotee. Swami Kriyananda, a direct disciple of Yogananda and Ananda’s founder discussed this topic many times. Here is a quote from a talk he gave in 1995.
“Master used to say that, ‘If you do even 100th of the things that I’ve done, that I’ve taught, you will find God.’ He, and the great masters that went before him, set this example of that intense kind of bhav, that intense kind of devotion, effort, in order to encourage us to meditate even a little bit. What we have to do is our very best. When Master said that 25% of the path is your effort, 25% is the guru’s effort and 50% is the grace of God he also meant that the 25%, which is your effort, is a 100% of your effort. You have to put out all that you have in order to find that goal.”
These percentages are not simply material facts that you might find in a statistical table. The path to liberation is not a mechanical formula. Spiritual teachings are available to everyone but it takes readiness on the part of the disciple to truly receive them and apply them to his life. In fact each of the teachings of a true path have as their central element God realization and the flawless practice of that teaching will lead the devotee to that realization. The devotee’s openness and receptivity are necessary to understand and truly use the teachings. In our soul’s long evolution toward liberation it takes many lifetimes to reach that stage where we realize we want nothing but God realization, no worldly attainment will do. The devotee must move beyond the stage of spiritual dabbling in various paths (often a necessary early step on the spiritual path) to complete commitment to a path, commitment to faithfully follow its practices and attunement to the Guru. This is the 25%. Then the devotee is ready to receive the Guru’s guidance, the Guru’s effort on behalf of the devotee, and God’s grace, the 75%. Many souls reach the point where they are ready for the guru and the spiritual path that will take them to liberation but still much ‘cleaning house’ is required before their consciousness is ready for ultimate liberation. Hence, once on a path the devotee must be steady in their commitment and practice regardless of the tests given to them by God.
When the disciple has reached that stage of total commitment and attunement to the guidance of the Guru (his 25%) then indeed doing even 100th of what the Guru teaches will take the devotee to God.