Observe fast for three days or give a quantity of alms enough to feed six needy persons or offer sacrifice (of an animal).

This hadith is about the Prophet Muhammad's advice to Ka'b b. 'Ujra when he was troubled by lice falling from his head. The Prophet suggested that he should shave his head and compensate for it by fasting for three days, giving alms or offering a sacrifice of an animal available to him. This hadith teaches us the importance of compensating for our troubles through acts of worship such as fasting, giving charity or sacrificing animals in order to please Allah SWT and gain His blessings in return. It also shows us how merciful the Prophet Muhammad PBUH was towards others; even though shaving one's head is not obligatory upon Muslims, the Prophet still advised Ka'b to do so out of mercy and kindness towards him so that he could be relieved from his suffering due to lice infestation on his scalp.