this verse from the Devahuti-stava in Padma Purana, quoted in HBV 4.358:
aradhananam sarvesam
visnor aradhanam param
tasmat parataram devi
tadiyanam samarcanam

Parvati asks Siva: “Of all kinds of worship, whose worship is best?” Siva replies, “The worship and devotional service of Lord Narayana, Visnu, is the highest.” Devi, disappointed, thinks, “But I am serving Siva, so I hold a lower position.” But then Siva, smiling says: “But higher than the worship of Narayana is worship of the devotees of Lord Narayana. That is even greater than devotion to the Lord Himself.”