If we revisit the passage the idea of 'rest' coming from the gospel is itself a non-canonical gospel saying:
The 'seeking' 'finding' and 'resting' with a secret source in Egypt is developed from a saying in a secret gospel repeated referenced by Clement."He that seeks will not rest until he finds; and he that has found shall marvel; and he that has marvelled shall reign; and he that has reigned shall rest."
οὐ παύσεται ὁ ζητῶν, ἕως ἂν εὕρῃ· εὑρὼν δὲ θαμβηθήσεται, θαμβηθεὶς δὲ βασιλεύσει, βασιλεύσας δὲ ἐπαναπαήσετα
When I came upon the last (he was the first in power), having tracked him out concealed in Egypt, I found rest (ὑστάτῳ δὲ περιτυχὼν (δυνάμει δὲ οὗτος πρῶτος ἦν) ἀνεπαυσάμην, ἐν Αἰγύπτῳ θηράσας λεληθότα).
The idea that 'rest' is given by scripture is explicit elsewhere in Stromata 1:
To these, then, and certain others, especially the Marcionites, the Scripture cries, though they listen not, "He that heareth Me shall rest with confidence in peace, and shall be tranquil, fearless of all evil."
Τούτοις τε οὖν αὐτοῖς καὶ ἑτέροις τισί, μάλιστα δὲ τοῖς ἀπὸ Μαρκίωνος ἐμβοᾷ οὐκ ἐπαΐουσιν ἡ γραφή· ὁ δὲ ἐμοῦ ἀκούων ἀναπαήσεται ἐπ' εἰρήνης πεποιθώς, καὶ ἡσυχάσει ἀφόβως ἀπὸ παντὸς 2.8.39.2 κακοῦ