वेदांतसारसंसिद्धं प्रणवार्थे प्रकाशनात् । एवमेव पुरा पृष्टो मंदरे नंदिकेश्वरः १६ ।
This has been explained in the meaning of the Praṇava (Om), the essence of Vedānta, by Nandikeśvara when asked by Sanatkumāra, the intelligent son of Brahmā, at the mountain Mandara.
बेरमात्रं च पूजार्थं श्रुतं दृष्टं च भूरिशः । शिवमात्रस्य पूजायां लिंगं बेरं च दृश्यते १८ ।
The embodied form alone is often observed in the worship of the deities other than Śiva. But both the phallic and the embodied forms are seen only in the worship of Śiva. Hence O benevolent one, please tell me precisely making me understand the truth.
O sinless one, since you are pious I shall tell you what Śiva Himself has said. Since Śiva has the bodiless aspect in virtue of His being the supreme Brahman, the Niṣkala liṅga, in conformity with the Vedic implication, is used only in His worship.