The proof is found in Revelation 11:19:
And the temple of God was opened in heaven, and there was seen in his temple the ark of his testament: and there were lightnings, and voices, and thunderings, and an earthquake, and great hail.
The word “testament” is the Greek word diatheke and is frequently translated covenant in the New Testament (see for example Luke 1:72, Act 3:25, 7:8, and Romans 11:27).
What was in the Ark of the testament, or covenant? The Ten Commandments:
(16) And thou shalt put into the ark the testimony (the 10 Commandments) which I shall give thee.
John’s seeing of the Ark of the Testament in heaven shows that the Ark that was made upon the earth, which contained the 10 Commandments, was a model of this heavenly one. It would not make much sense to say that the Ark in heaven was empty of this law, for, what purpose would that serve? And why else is it called the Ark “of his testament?”
Note also that when John saw the Ark of the Testament in the heavenly temple, there immediatly followed… “thundering, lightings, and an earthquake.” Similar events took place when God gave the 10 Commandments at Mount Sinai (see Exodus 20:18). This is yet more evidance of the fact that the 10 Commandments are present in his vision.