This quote is found in 8th Patala of Karana Agama:
देव्युवाच:
Devi asked:
देवदेव जगन्नाथ सर्वानुग्रहतत्पर।
विधानमिदमुकृष्टं वेदमन्त्र्समन्वितम्।।
वेदाधिकारिणां सम्यगनुष्टेयं प्रकाशितम्।
स्रीमुख्याः केन विधिना कृतार्थाः स्युर्निबोध मे।।
Oh The Gods of Gods, the Lord of World, You are ready to grace all. This well-mannered method is connected with the hymns of Vedas. Its practice of the learned of Vedas has been made known in well-manner. Reveal to me by which method women etc. achieve their object of desire.
महादेव उवाच:
Mahadeva answered:
महादेवि प्रवक्ष्यामि तान्त्रिकं पूजनक्रमम्।
अनुष्ठितेन येन स्यु: सर्वे सर्वार्थभाजना:।।
विद्यासु श्रुतिरुत्कृष्टा रुद्रैकादशिनी श्रुतौ।
तत्र पञ्चाक्षरस्तत्र शिव इत्यक्षरदूयम् ।।
Oh, Mahadevi! I would reveal a method of Tantrik worship of which performance all and the complete Arthas achieved. Shrutis/Vedas are the best and excellent among all Vidyas (sources of knowledge); In Shrutis also Sri Rudram hymn is the Supreme, in Sri Rudram also Panchakshara is Supreme and in the Panchakshara also the two letters 'Shiva' is Supreme.