SB 6.16.33
tatah samadhaya mano manisaya
babhasa etat pratilabdha-vag asau
niyamya sarvendriya-bahya-vartanam
jagad-gurum satvata-sastra-vigraham
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Thereafter, by controlling his mind with his intelligence and thus restricting his senses from external engagements, he recovered suitable words with which to express his feelings. Thus he began offering prayers to the Lord, who is the personification of the holy scriptures [the satvata-samhitas like the Brahma-samhita and the Narada-pañcaratra] and who is the spiritual master of all. He offered his prayers as follows.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
One cannot offer prayers to the Lord with mundane words. One must become spiritually advanced by controlling the mind and senses. Then he can find suitable words to offer in prayers to the Lord. Quoting the following verse from the Padma Purana, Srila Sanatana Gosvami forbids us to sing any song not sung by authorized devotees.
avaisnava-mukhodgirnam
putam hari-kathamrtam
sravanam naiva kartavyam
sarpocchistam yatha payah
The words or songs of a person not fixed in Vaisnava behavior, not strictly following the rules and regulations and chanting the Hare Krsna mantra should not be accepted by pure devotees. The words satvata-sastra-vigraham indicate that the sac-cid-ananda body of the Lord can never be accepted to be made of maya. Devotees do not offer prayers to the Lord in an imaginary form. The existence of the Lord’s form is supported by all Vedic literature.