Shiva Purana
आज्ञासिद्धं क्रियासिद्धं श्रद्धासिद्धं ममात्मकम् ॥ एवं चेद्दक्षिणायुक्तं मंत्रसिद्धिर्महत्फलम् ॥ २ ॥
If a mantra is well practised with the acquisition of behest, attended with holy rites, equipped with faith in me and accompanied by fee it is greatly efficacious.
english translation
AjJAsiddhaM kriyAsiddhaM zraddhAsiddhaM mamAtmakam ॥ evaM ceddakSiNAyuktaM maMtrasiddhirmahatphalam ॥ 2 ॥
hk transliteration by Sanscript