There is a story in Narada Purana which narrates the greatness of Tulsi leaves and how it saved a man from certain death. There lived a man named Madhava who never performed any worship in his entire life and neither did he do any good deeds.
Madhava cultivated his land and kept cows for a living. Every day, be brought grass for his cows from his field.
One day, while collecting grass for his cows, he cut some Tulsi leaves along with the grass. He was destined to die that evening and a snake came slithering towards him to bit him and end his life at the predetermined time. But the snake was unable to bite him as he had Tulsi leaves in his hands.
The man picked up the bundle of grass along with the Tulsi leaves and placed the bundle on his head and returned home. The snake hid in the bundle, hoping to bite him when he is not contact with the Tulsi leaves.
Neighbor of Madhava was Madhusudana, a pious man. He was able to see the snake in the grass bundle and through his yogic powers he also saw the attendants of Yama (Yamadutas) following him.
Madhusudana realized that the snake was there to bite his neighbor and Yamadhutas were there to take away his life the moment he was bitten by the snake.
Madhusudana asked the Yamadutas as to how he could save the life of Madhava, his neighbor.
Yamadutas told Madhusudana that he can save the life of his neighbor if he is ready to transfer the benefits of Tulsi puja of the day to his neighbor. Madhusudana performed Tulsi puja daily and he was ever ready to help his neighbor.
Madhusudana transferred that day’s merits of Tulsi Puja to Madhava and his life was spared.
Madhusudana then informed Madhava what had happened and advised him to do Tulsi puja every day. From then on, Madhava too did tulsi puja every day.
Srimad Bhagavatam and Narasimha Purana states that the very best offering to Bhagavan Vishnu is Tulsi leaves.