In Vamana Purana, Sage Pulastya explained Narada about the significance of Sarasvat stotra to attain Vaikunta.
Demon’s curiosity to know about penance
Once there lived a Kshatriya who kept some grudges with Brahmins. After his death he was reborn as a demon. He spent his life tormenting others for many years. One day he saw a hermit doing penance and wanted to trouble him, but he failed to go nearer the Hermit. Surprised by this he waited for the hermit to open his eyes.
The Hermit opened his eyes after four months and shocked to see a sick demon sitting in front of him. When hermit asked the reason for his presence, demon replied that he wanted to learn what he(Brahmin) were doing and want to know what force is protecting him(Brahmin). The hermit refused to answer the demon and advised to ask some other Brahmin.
Demon kept on his search for another Brahmin and found one who is doing Penance and held the Brahmim, asking to help him learn about what they are doing. The Brahmin got scared and told demon that he would return after giving some fruits to his teacher. Demon dint allow him as he thought the Brahmin might run away.
Demon attains vaitunta(Vishnu Lokha)
In this desperate situation that Brahmin turned towards the burning fire which is nearby him and pleaded for help. Soon Goddess Saraswathi emerged from it and told the Brahmin to teach the demon the “saraswathi stotra”.
The Brahmin then gave the teaching of the “Saraswat Vishnu stotra” which is capable of liberating all the sins, to the demon an told him to recite the mantra, three times a day with some conditions.
The demon then went to a place called Shaligram and started doing his penance which made him not only free from his sins but also the demon attained vaikunta(Visnu Lokha).