To inspire His countrymen to serve the poor masses of India, Swami Vivekananda coined a beautiful term 'Daridra-Narayana'. Can anyone provide a scriptural shloka or instruction that contains the spirit of the word?
Yes, the spirit of the word coined by Swamiji is fully supported by our holy scriptures.
The word 'Daridra-Narayana' is samasa with the vyasa-vakya : Daridra-rupi-Narayana, ie the Lord Himself in form of poor. It is a (Madhyapadalopi) Karmadharaya samasa.
Sri Bhagavan says in the Srimad-Bhagavatam:
I am present in every living entity as the Supersoul. If someone neglects or disregards that Supersoul everywhere and engages himself in the worship of the Deity in the temple, that is simply imitation.
One who worships the Deity of God in the temples but does not know that the Supreme Lord, as Paramātmā, is situated in every living entity’s heart, must be in ignorance and is compared to one who offers oblations into ashes (Reference :http://gitabase.com/eng/SB/3/29)
Swamiji says the same thing in His famous poem translated as : To a Friend:
These are His manifold forms before thee,
Rejecting them, where seekest thou for God?
Who loves all beings without distinction,
He indeed is worshipping best his God.
Sri Bhagavan also orders to prostrate before all beings in Srimad-Bhagavtam.
'Sarvam Vishnumayam Jagat' (Everything in this world are a manifestation of Lord Vishnu), 'Yatra yatra jivas tatra tatra Shivah' (Every being is a manifestation of God) are very well-known sayings. The Destroyer has also mentioned 'Manava-seva is Madhava-Seva'.
In other scriptures, like Gita and Viveka-Chudamoni also, seeing God in all beings and working for the welfare of all beings are highly praised. For example,nthe Gita says:
12.4 By fully controlling all the organs and always being even-minded, they, engaged in the welfare of all beings, attain Me alone.
A beautiful incident from the 'Swami-Sishya-Samvad' by Saratchandra Chkraborty:
After returning from Eastern Bengal Swamiji stayed in the Math and lived a simple childlike life. Every year some Santal labourers used to work in the Math. Swamiji would joke and make fun with them and loved to hear their tales of weal and woe. One day several noted gentlemen of Calcutta came to visit Swamiji in the Math. That day Swamiji had started such a warm talk with the Santals that, when he was informed of the arrival of those gentlemen, he said, "I shan't be able to go now. I am happy with these men." Really that day Swamiji did not leave the poor Santals to see those visitors.
One among the Santals was named Keshta. Swamiji loved Keshta very much. Whenever Swamiji came to talk with them, Keshta used to say to Swamiji, "O my Swamiji, do not come to us when we are working, for while talking with you our work stops and the supervising Swami rebukes us afterwards." Swamiji would be touched by these words and say, "No, no, he will not say anything; tell me a little about your part of the country" — saying which he used to introduce the topic of their worldly affairs.
One day Swamiji said to Keshta, "Well, will you take food here one day?" Keshta said, "We do not take food touched by you; if you put salt in our food and we eat it, we shall lose our caste." Swamiji said, "Why should you take salt? We will prepare curry for you without salt, will you then take it?" Keshta agreed to it. Then at orders of Swamiji, bread, curry, sweets, curd, etc. were arranged for the Santals, and he made them sit before him to eat. While eating, Keshta said, "Whence have you got such a thing? We never tasted anything like this." Feeding them sumptuously, Swamiji said, "You are Nârâyanas, God manifest; today I have offered food to Narayana." The service of "Daridra Narayana" — God in the poor — about which Swamiji spoke, he himself performed one day like this.
After their meal, the Santals went for rest, and Swamiji, addressing the disciple, said, "I found them the veritable embodiment of God.
Another quotation from His complete works is worth mentioning:
May I be born again and again, and suffer thousands of miseries so that I may worship the only God that exists, the only God I believe in, the sum total of all souls — and above all, my God the wicked, my God the miserable, my God the poor of all races, of all species, is the special object of my worship.
By the way, I had asked the question because I heard a handful criticising Swamiji for coining the term 'Daridra-Narayana' by saying that 'Lakshmipati' (the Husband of the Goddess of Wealth) Narayana can never be 'Daridra' (poor)!!! It seems they neither got the opportunity to study samasa nor to read our scriptures minutely.