Richard Steiner says in "Ancient Hebrew" (in The Semitic Languages, ed Robert Hetzron, Routledge 1997):
A process which serves some of the same functions as the one which creates genitive phrases and which sometimes alternates with it is the insertion of the preposition le- 'to, belonging to' (eg 2. Kings 5:9, Ruth 2:3), usually preceded by relative 'ašer/še-.
He gives biblical examples of 'ašer le- but not of še le-, but he has mentioned the latter possibility, and he gives examples of šl- from the Dead Sea Scrolls and the tomb of Benei Hezir.
He goes on to say
In [Mishnaic Hebrew], the phrase še+le- 'that belongs to' has been reanalyzed as a single morpheme, a new preposition šel with the meaning 'of'
(I have not reproduced his precise but unfamiliar transliteration scheme).