Acts 5:1-16

One couple sold some land and gave a portion of the money to the apostles, but told them it was all of it. Peter asked the man why he tried to deceive them and told him the lie was against God and not men. When the man heard this, he fell down dead. Later, when his wife came, Peter asked her if the sum brought by her husband was the full amount they had received for the land. She said that it was and she fell dead as well.

The sin of this couple was not that they didn't give the full amount they had received for the land. Their sin was in the fact that they tried to lie about what they were doing. Chances are they were trying to appear holy by pretending they had given everything like others had done, but secretly holding on to a portion to feel secure. Thus, they didn't trust God to take care of their needs and felt they needed to have something to fall back on. We can't fool God. He knows when we are showing off or trying to look like we're completely devoted to Him while secretly holding something back. God only honors those whose motives are pure. When we're just trying to look good, He sees right through it.