Conversation so far:
What was the Guinness World Records for Mohammed Rafi?
Rafi had challenged the claim that Lata Mangeshkar has recorded the highest number of songs (not less than 25,000 according to Guinness).
How many songs did Mohammed Rafi sing?
They estimated the number of songs sung by Rafi to be anything from 25,000 to 26,000.

Question: What did he write in the letter?
Self-contained rewrite: What did Mohammed Rafi write in the letter to Guinness World Records?