Shiva Purana
अकारो बीजमाख्यातमुकारः शक्तिरुच्यते ॥ मकारः कीलकं प्रोक्तं मोक्षार्थे विनियुज्यते ॥ ६२॥
The letter “A” is the seed; the letter “U” is Śaktī; the letter “M” is Kīlaka. It is used for the sake of liberation.
english translation
akAro bIjamAkhyAtamukAraH zaktirucyate ॥ makAraH kIlakaM proktaM mokSArthe viniyujyate ॥ 62॥
hk transliteration by Sanscript