SB 8.18.28
tat-pada-saucam jana-kalmasapaham
sa dharma-vin murdhny adadhat sumangalam
yad deva-devo girisas candra-maulir
dadhara murdhna paraya ca bhaktya
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Lord Siva, the best of demigods, who carries on his forehead the emblem of the moon, receives on his head with great devotion the Ganges water emanating from the toe of Visnu. Being aware of religious principles, Bali Maharaja knew this. Consequently, following in the footsteps of Lord Siva, he also placed on his head the water that had washed the Lord’s lotus feet.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Lord Siva is known as Ganga-dhara, or one who carries the water of the Ganges on his head. On Lord Siva’s forehead is the emblem of the half-moon, yet to give supreme respect to the Supreme Personality of Godhead, Lord Siva placed the water of the Ganges above this emblem. This example should be followed by everyone, or at least by every devotee, because Lord Siva is one of the mahajanas. Similarly, Maharaja Bali also later became a mahajana. One mahajana follows another mahajana, and by following the parampara system of mahajana activities one can become advanced in spiritual consciousness. The water of the Ganges is sanctified because it emanates from the toe of Lord Visnu. Bali Maharaja washed the lotus feet of Vamanadeva, and the water with which he did so became equal to the Ganges. Bali Maharaja, who perfectly knew all religious principles, therefore took that water on his head, following in the footsteps of Lord Siva.