1 Peter 1:16 because it is written, “Be holy, for I am holy.”
When we talk to other people about God and about Christ we should and must rely on the Holy Spirit to guide us through our conversations.
These words were used here because the apostle knew they would make a difference to the people he was talk to; they knew the scriptures and reciting verses from the scriptures strengthens their trust and belief.
We may not know where to look to find this in the Old Testament (actually Leviticus 11:44) but God can lead us to say things to others that will touch their hearts.
God is here with us every step of the way. Trust Him.