I have heard this (with the number varying by each account, e.g., to 10 or 20), and heard first-hand accounts of Hands of the Cause repeating this, but I know of no authentic text (nor even pilgrim's note) to this effect.
In this compilation
, there are some quotations on the importance of a number of different secular areas of study, but while several indicate the need for engaging in secular study, none of these are from the point of view of urging more or enough secular study in comparison to spiritual study. On the contrary, a good number urge the need to take enough time for spiritual study, though given that both types of study are encouraged in this quotations, it would only stand to reason that it would be possible for one to take too little time for secular study.
There is also this quote which indicates at least one case where intellectual preparation is seen as a prerequisite for success in teaching:
"As to teaching work in colleges and universities, this is very important, for students as a whole are open-minded and little influenced by tradition. They would easily enter the Cause if the subject is properly presented and their intellect and sentiments properly satisfied. This, however, should be attempted only by persons who have had university training and are, therefore, acquainted with the mind of the intelligent and educated youth..."
(From a letter written on behalf of Shoghi Effendi to an individual believer, February 3, 1932)
And there is also the following which might point to the need for balance in our studies:
"You ask about 'spiritual indigestion': Bahá'ís should seek to be many-sided, normal and well-balanced, mentally and spiritually. We must not give the impression of being fanatics but at the same time we must live up to our principles."
(From a letter written on behalf of Shoghi Effendi to an individual believer, March 12, 1946: Living the Life)
(Note, however, that I also recall encountering another quote or pilgrim's note of 'Abdu'l-Baha which indicated in a joking manner that such "indigestion" (as also metnioned in the just-mentioned quote) was something to be overcome.)