Shiva Purana
युवाभ्यां हि मया दत्ता कारणत्वप्रसिद्धये ॥ मंत्ररत्नं च सूत्राख्यं पञ्चाक्षरमयं परम् ॥ ७६ ॥
I have given it to you both to prove that you are the cause. The gem of mantras is also known as the Sutra and is composed of five syllables.
english translation
yuvAbhyAM hi mayA dattA kAraNatvaprasiddhaye ॥ maMtraratnaM ca sUtrAkhyaM paJcAkSaramayaM param ॥ 76 ॥
hk transliteration by Sanscript