Assalaamu alaykum. I want to know the grade of authenticity of the hadith which states that uttering a special supplication after the ‘Asr prayer on Friday will wipe out 80 years of sin. May Allah reward you.

All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger. 

This hadeeth is a fabricated lie. Shaykh Al-Albaani mentioned it in Silsilat Al-Ahadeeth Adh-Dha'eefah wa Al-Mawdhoo'ah, no. 215.

Its wording is as follows: "Whoever sends blessings upon me eighty times on Friday, his sins will be forgiven for eighty years." He was asked: "How do we send blessings upon you, O Messenger of Allah?" He said, "Say: 'O Allah, send blessings upon Muhammad, Your slave, Your Prophet, and Your Messenger, the unlettered Prophet’, and count it as one."

Then Shaykh Al-Albaani ruled that it is fabricated and mentioned the reason for that.

Allah knows best.