The Pleading Woman - 58:4
Whoever cannot find a slave to free should fast for two consecutive months before sexual intercourse. And he who cannot afford it should feed sixty poor people. You must believe in Allah and His messenger. These are the limits set by Allah. The disbelievers are doomed to a painful punishment.
Two months of fasting is a punishment for what other sin?
The Women/4:92
A believer shall not kill a believer except by accident. Whoever kills a believer by accident must release a believing slave and pay a diet to the family of the deceased. Unless they give up the diet and accept it as charity. If it is a believer from a community that is at war with you, then you must release a believing slave. But if the deceased belongs to a community with whom you have an agreement, then you must give his family a diet and release a believing slave. Whoever cannot find the money or a slave to release should fast for two months in a row so that Allah may accept his repentance. Allah is All-Knowing, All-Wise.