The command of Isaiah. We cannot be saved by works alone. We need faith in Christ, but our faith lacks sincerity if it doesn't reach out to others.
Fasting. Some denominations discourage it. God calls upon us to fast from time to time. It can be beneficial spiritually and physically, but it only helps the person doing it. God wants our service to go beyond our own selves, beyond our own personal growth. He wants us to perform acts of charity, kindness, justice and generosity. Pleasing God is more than what we eat or don't eat. It is what we do for others and for Him that matters most. Those are our "works".
Certainly, we can fast in prayer for others. Jesus called us to be active Christians, not passive ones. He called us to reach out to the poor and homeless, the sick and the lonely not only with prayer but by sharing the blessings God has bestowed upon us. It's not only putting a tithe in the collection plate on Sunday - sometimes it's preparing a meal for the neighbor down the street, or helping someone to their car with groceries. Sometimes it's giving your last dollar to help someone else.
You know the verse: "God loves a cheerful giver" (2 Cor 9:7) - give with your heart; give of yourself!