There is an interesting story related to Lord Vishnu’s mantra.
Lord Vishnu is described as Shantakaram; always peaceful and steady minded.
Once Sage Kashyapa was celebrating a yajna and all the sages had gathered to witness the same. Now there arose a doubt in their mind as to who was highest of all the divinities. Sage Narada replied that the Trinity – Brahma, Vishnu and Maheswara were the highest of all, but Lord Vishnu was Supreme. Either praise or blame never disturbed him. He was every peaceful and in bliss. The sages asked Narada to prove it. Narada called sage Bhrigu aside, whispered something in his hear. Bhrigu was sent to verify the truth of Narada’s statement.
He first went to Brahmaloka. He saw that Lord Brahma was busy in his act of Creation. From afar, Bhrigu started condemning Lord Brahma. He said “Oh! Brahma you don’t have the proper knowledge of creation! Your creation is full of defects and faults. Therefore all your efforts are in vain. Not a single things in your creation is worth praising”. Lord Brahma was annoyed to hear these words. He got up from his seat and came forward to punish Bhrigu. But the sage ran away from Brahmaloka. Then Bhrigu went to Shivaloka. He saw Lord Shiva engaged in Tandava Nritya. From a distance Bhrigu began to criticise Lord Shiva. He said “Oh! You are not looking after your department properly at all. Your task is to destroy the evil of the world; but the world today is full of evil. You are busy dancing the whole day and are not performing the task allotted to you, why should we worship you then?” Lord Shiva was angry to hear this and he was about to open his third eye to burn the sage, but the sage ran away from there.
Then he went to Vaikuntha. He saw Lord Vishnu resting upon the Sesha. He got very angry seeing that Lord was not coming for his devotees but seemed to be engaged in self-enjoyment; he went near Lord Vishnu and kicked hard upon his chest. Then Bhrigu was afraid of the consequences and was about to run away. But to his surprise he saw Lord Vishnu getting up from his seat falling at his feet. The Lord said, “Oh sage! please forgive me for not noticing you and ignoring you, thereby causing anger to rise in you; My iron-like chest may have injured your foot. Let me press it and comfort you”. What great tolerance! What peaceful state! Bhrigu was taken back to hear these words and asked the Lord to forgive him.
Then the Sage came back to the council of the sages at the yajna and narrated the whole story. Hearing it, all the sages believed that Lord Vishnu alone was Shantaakaram and their doubt was cleared.
Read more about the mantra here: