Āmalakī Ekādaśī 2159

Sri Krishna told Yudhishthira what the sage Vasishtha once told King Mandhata. The amalaki (amla) tree, Vasishtha said, appeared at the beginning of creation and is dear to Vishnu. Vishnu dwells at its root, Brahma above it and Rudra in its trunk, and its worship destroys all sins.

In the Brahmanda Purana telling, a king of Vaidisha and all his people kept this Ekadashi of Phalguna Shukla by worshipping the amla tree and Vishnu through the night. A hungry hunter came upon them, watched the worship and heard the stories while fasting and awake. In his next life he was born as King Vasuratha, and when enemies attacked him, a divine power came forth from his body and protected him.

Devotees fast, worship Vishnu beside an amla tree, offer water and lamps to it, and keep vigil. The merit is said to surpass the gift of a thousand cows.