Skandapurana 6.3.83.109

6.3.83.109

lakṣmīśca gomaye nityaṃ pavitrā sarvamaṅgalā .
gomayālepanaṃ tasmātkartavyaṃ pāṇḍunandana .. 83.109 ..

And Lakṣmī, pure and all-auspicious, is always in cow dung; / therefore anointing with cow dung must be performed, O son of Pāṇḍu.