All the śāstras and mahā-jana vāṇīs loudly proclaim (with the loud sounds of the Dundubhī-drum of glory) the glories of serving the bhaktas.

In Śrīmad Bhāgavata, Śrī Vidura Mahāśaya tells Śrī Maitreya Muni —

yat sevayā bhagavataḥ kūṭasthasya madhudviṣaḥ
ratir āso bhavet tīvraḥ pādayor vyasanārdanaḥ (3.7.19)

“O Muni! By serving all these devotees of the Lord, deep love for Lord Śrī Madhusūdana, Who is immutable and Who destroys the miseries of material existence, is brought about.”

Śrīmat Jīva Gosvāmīpāda has written in his commentary on this verse in Bhakti Sandarbha (244 anu):

tīvra iti viśeṣaṇaṁ prasaṅga mātrāt paricaryāyāṁ viśiṣṭaṁ phalaṁ dyotayati –  “Service is of two kinds – paricaryā and prasaṅga. Giving monetary donations or eatables to the saints or massaging their feet is called paricaryā sevā and making them hear the topics or the holy names of Śrī Hari is called prasaṅga sevā.”

Of the two, paricaryā sevā is again shown to be the more glorious one. As a result of this, one swiftly becomes qualified to receive the treasure of love of God.
Śrī Bhagavān Himself told Śrī Uddhava:

mad bhakta pūjābhyādhikā – “I am more pleased by a devotee who worships My devotee than by a devotee who worships Me directly.”

A person who serves the Vaiṣṇavas should take care that he sees the Vaiṣṇavas as Śrī Viṣṇu Himself! Even if a Vaiṣṇava is low born or ill-behaved, he should be duly served, respected and bowed down to. If any kind of disrespect is shown to a Vaiṣṇava who wears the signs of God, it will be an offence. When the Vaiṣṇavas are served, it must be done to all Vaiṣṇavas, regardless of caste, birth or behaviour, but association must only be had with the bona fide bhaktas who are pure in conduct (sadācārī). It is not relishable to associate with ill behaved or impure bhaktas — this is the verdict of the śāstras.

In Śrīmad Bhāgavata, Lord Śrī Ṛṣabha-deva instructs His sons as follows:

mahat sevām dvāram āhur vimuktes tamo-dvāraṁ yoṣitāṁ saṅgi saṅgam
(Bhāg. 5.5.2) —
“The gateway to liberation or prema bhakti is the service of the great saints and the company of men attached to women is the gateway to hell.” In the Padma Purāṇa it is seen:

ārādhanānāṁ sarveṣāṁ viṣṇor ārādhanaṁ param
tasmāt parataraṁ devi tadīyānāṁ samarcanam

Śrīmān Mahādeva said: “O Devi! Of the worship of all gods and goddesses, the worship of Viṣṇu is the greatest, and even greater than that is the worship of the Vaiṣṇavas!”

In order to please Śrī Viṣṇu, the Vaiṣṇavas must always be satisfied — tasmād viṣṇu prasādāya vaiṣṇavān paritoṣayet (Itihāsa Samuccaya).

In Śrī Caitanya Bhāgavata, it is described —

kṛṣṇa sevā hoite-o vaiṣṇava sevā boḍo; bhāgavata ādi sab śāstre koilo doḍo
eteke vaiṣṇava sevā parama upāya; bhakta sevā hoite sabāi kṛṣṇa pāya
kṛṣṇa bhajibāra yāra āche abhilāṣa; se bhajuk kṛṣṇera maṅgala nija dāsa
sabāre śikhāya gauracandra bhagavāne; vaiṣṇavera sevā prabhu koriyā āpane

“Śrīmad Bhāgavata and other scriptures firmly proclaim that the service of the Vaiṣṇavas is even greater than the service of Kṛṣṇa. Thus the service of the Vaiṣṇavas is the greatest means for everyone to attain Kṛṣṇa. Those who desire to worship Kṛṣṇa should worship Kṛṣṇa’s own auspicious devotee. Gauracandra Bhagavān has taught that to everyone by personally engaging in the service of the Vaiṣṇavas.”

Svayaṁ Bhagavān Śrīman Mahāprabhu showed all the people of the world how nice it is to serve the devotees by giving a personal example. Therefore, anyone who desires bhakti must certainly engage in the service of the bhaktas. Śrīla Vṛndāvana dāsa described Mahāprabhu’s service of the bhaktas as follows —

niṅāḍaye vastra kāro koriyā yatane; dhuti vastra tuli kāro den to āpane
kuśa, gaṅgā mṛttikā kāhāro den kore; sāji bohi kon dina cole kāro ghare
sakala vaiṣṇava-gaṇa ‘hāy hāy’ kore; ‘ki koro ki koro’! tabu kore viśvambhare

“He carefully wrung out the (wet) garments of one bhakta and handed him a dry dhoti, he handed Kuśa grass and clay from the Gaṅgā to another, and one day he went to the house of one bhakta carrying a tray for worship. All the Vaiṣṇavas exclaimed “Alas, alas! What are You doing?!”, but still Viśvambhara would do these things.”

The Lord also revealed for which reason He was serving His own devotees —

tomarā se pāro kṛṣṇa bhajana divāre;
dāsere sevile kṛṣṇa anugraha kore
tomā sabā sevile se kṛṣṇa bhakti pāi etc.

“You can bestow Kṛṣṇa-bhajana — when I serve His servants, Kṛṣṇa will bestow His mercy upon Me. When I serve you all I will get Kṛṣṇa-bhakti.”

In his Śrī Bhakti Rasāmṛta Sindhu-grantha, Śrīmat Rūpa Gosvāmīpāda has written —

yāvanti bhagavad bhakter aṅgāni kathitānīha
prāyas tāvanti tad bhakta bhakter api budhā viduḥ (1.2.219)

“Each item of devotion to the Lord that has been described here is exceeded by the corresponding item of devotion to His devotees. The wise men understand this.”

Here one may ask: “How will the service of the Vaiṣṇavas that the scriptures and the mahājanas are promulgating be accomplished? After all, it is the Lord who is worshipped through the process of hearing, chanting etc. . How can the worship of the Vaiṣṇavas also be considered a limb of bhakti?”

To answer this question, the Gosvāmīs have said – “All the items of the Lord’s bhajana that have been mentioned, such as hearing, chanting and remembering are exceeded by their corresponding items of worship of the Vaiṣṇavas.”

That means one must hear and chant the glories of the Vaiṣṇavas, offer them formal worship and praises, see them, offer obeisances unto them, circumambulate them and pray to them with the greatest devotion, because devotion to the Vaiṣṇavas is non-different from devotion to Kṛṣṇa.

The Śrī Nārada Pañcarātra said: vaiṣṇavānāṁ parābhaktiḥ — “One must be most devoted to the Vaiṣṇavas.”

– Rādhākuṇḍa Mahānta Paṇḍita Śrīmat Ananta Dāsa Bābājī Mahārāja