This incident happened after Swamiji had returned after his great address at world parliament of religion. He was wandering near Himalayas some time. In course of the journey he came across a river. The boat had already left the bank so he sat by the shore and waited for the boat to come back. As he was waiting he saw another Sadhu (monk) approaching.
The other saint saw Swamiji sitting and asked why he was sitting. To this he replied he was waiting for the boat.
The other Sadhu - What's you name?
Swamiji - I am Vivekananda.
The other sadhu said - Oh! You are the famous Vivekananda (said in a mocking tone) who thinks just by speaking in foreign he has become a great saint.
Could you cross this river? See I can cross the river by walking on it and he demonstrates his power by crossing the river on his foot.
Swamiji showing his respect tells him indeed its a great power and asks him humbly how much time did the saint take to get this power?
The other Saint (feeling proud) - its not so easy, I had to bear the tough Himalayas for 20 years, devoted myself to solitude and after 20 tough years of extreme "Sadhana"/ worship I got this power.
To this Swamiji said - you wasted 20 years to learn something that a boat would help me do in 5 minutes. You could have devoted these 20 years serving the destitute the poor. You wasted 20 years to save 5 mins, its not wisdom.
The other saint was left speechless and true Sadhana lies in service of the poor the destitute.
Swamiji always said and did "Jiva seva is Shiva seva" ie Service to people is Service to God.