Conversation so far:
What were the earliest sources on the Battle of Kosovo with Miloš Obilić?
The earliest sources on the Battle of Kosovo with Miloš Obilić, which generally favour the cult of Prince Lazar, do not mention Miloš or his assassination of the sultan.
What was the assassination of the sultan that the earliest sources on the Battle of Kosovo, which generally favour the cult of Prince Lazar did not mention?
The assassination of sultan Murad and one of his sons was also mentioned in the instructions of the Venetian Senate issued to Andrea Bembo on 23 July 1389
What about the assassination of sultan relates to Miloš Obilić?
Miloš Obilić features prominently in later accounts of the Battle of Kosovo as the assassin of the Ottoman sultan Murad I.

Question: When was the assassins first appearance in the earlier years?
Self-contained rewrite: When was the assassin of the sultans' first appearance in the earlier years in the earliest sources of Miloš Obilić?