Deuteronomy 3:2
And the Lord said unto me, Fear him not: for I will deliver him, and all his people, and his land, into thy hand; and thou shalt do unto him as thou didst unto Sihon king of the Amorites, which dwelt at Heshbon.
Cross-reference
Deuteronomy 3:11 describes Og's giant size, illustrating why the 'fear not' command was needed—he was a formidable foe.
Deuteronomy 3:6 describes the actual destruction of Og's cities, fulfilling the command given here and using the same 'as we did to Sihon' comparison.
Deuteronomy 2:24-37 records the defeat of Sihon, the very precedent God commands Moses to follow when facing Og in this verse.
Deuteronomy 2:33 recounts God giving Sihon over to Israel, the very victory that serves as the pattern for the promised defeat of Og.
Deuteronomy 20:3 gives the general battle instruction 'do not fear'—the same principle applied specifically here against Og.
Numbers 14:9 echoes the 'do not fear' command—Caleb and Joshua urging Israel not to fear the Canaanites, as God is with them.
Numbers 21:23-25 is the original account of Israel defeating Sihon, the exact event God references as the model for dealing with Og.
In Acts 27:24, an angel tells Paul 'Do not be afraid' and promises deliverance—echoing the same divine reassurance given to Moses here.
Numbers 21:34 contains the almost identical command to Moses regarding Og—'Do not fear him, for I have given him into your hand'—making this the direct source.
2 Chronicles 20:17 commands 'Do not fear' in battle, promising God's deliverance—similar to the assurance given here.
Isaiah 41:10 expands the 'fear not' promise with God's presence and help—the same encouragement given to Moses here.
Isaiah 43:5 repeats 'fear not' with a promise of restoration—the same divine reassurance as in the command to Moses.
Acts 18:9 is a direct 'Do not be afraid' from the Lord to Paul—mirroring the same imperative given to Moses here.