Shiva Purana
वाचकेन समाहूतः पशून्मोचयते क्षणात् ॥ तस्माद्वाचकतासिद्धिः प्रणवेन शिवम्प्रति ॥ ४८ ॥
Invoked with the mantra he releases the Jīva instantaneously. Hence siddhi is achieved with the mantra Om for Śiva.
english translation
vAcakena samAhUtaH pazUnmocayate kSaNAt ॥ tasmAdvAcakatAsiddhiH praNavena zivamprati ॥ 48 ॥
hk transliteration by Sanscript