There are two such letters that wash away the sins of your seven births. There is such a mantra that Mahadev himself said, by chanting that mantra, the full fruit of chanting Vishnu Sahasrara Naam can be attained.
Those two letters are Ram.
And there is a special mantra
श्रीराम रामेति रामेति, रमे रामे मनोरमे.
सहश्रनाम तुल्यों, रामनाम बरानने.
This mantra is known as “Shri Ram Tarak Mantra”. And its chanting is equivalent to chanting the entire Vishnu सहश्रनाम or 1000 names of Vishnu. This mantra is also known as Shri Ram Raksha Altotrim.
Related Story:
Once Lord Shiva requested his wife Parvati to have a meal with him. Bhagavati Parvati escaped saying that he was reciting Vishnu Sahasranaam. After waiting for some time, when Shiv again called Maa Parvati, Maa Parvati replied that she was reciting Vishnu Sahasranaam, she would be able to come only after the completion of that recitation. Lord Shiva was in a hurry to eat food and the food was getting cold on him. That’s why Lord Shiva said – Parvati say Ram Ram. Once you say Ram Ram, you get the full fruit of Vishnu Sahasranaam. The name of Ram is equivalent to the name of Vishnu.
In this way, after hearing the name of these two letters from the mouth of Lord Shiva as Bishnu Sahasrnaam, mother Parvati Ram chanted the name of these double letters and then mother Parvati became very happy and took food with Shiva.