Moses and Aaron sin and are banished from the Promised Land (Numbers 20: 8 - 12)

"Moses, get your walking stick. They you and Aaron call the people together and command that rock to give you water. That's how you will provide water for the people of Israel and their livestock." Moses obeyed and took his stick from from the sacred tent. After he and Aaron had gathered the people, around the rock, he said, "Look you rebellious people, and you will see water flow from this rock!" He raised his stick in the air and struck the rock two times. At once, water gused from the rock, and the people and thier livestock had water to drink. But the LORD said to Moses and Aaron, "Because you refused to believe in my power, these people did not respect me. And so, you will not be the ones to lead them into the land I have promised."
In these verses Aaron accompanies Moses, so that both of them share in the sin of mistrusting God. The text does not say what their sin was exactly: presumably it was because they struck the rock twice due to lack of faith, instead of once or in the fact that they struck the rock whereas God had told them to speak to the rock. This is also a sin which is attributed to the people's disobedience.
Daily Prayer: Father I am indeed a rebellious man. The Bible teaches that I am born a child of wrath. Apart from you, I can do nothing. Thank you for Jesus Christ in whom I and my sins are hidden. I now rest on Christ as my basis for your love and blessing in my life. Not by my power, but by your grace I am saved. In Jesus name I pray Amen.