Agnipurana 184.3

184.3

bhūmiśāyī niśāyāñca śaṅkaraṃ pujayed vratī /
pauṣe śambhuṃ ghṛtaṃ prāśya māghe kṣīraṃmaheśvaram(1) //

Sleeping on the ground at night, the observer of the vow should worship Śaṅkara. / In Pauṣa [one should worship] Śambhu after consuming clarified butter, in Māgha Maheśvara [after consuming] milk;