An Nasai: Hadith 4164

"A man called out to the Prophet and said: 'We used to sacrifice the 'Atirah - i.e., during the Jahiliyyah - in Rajab; what do you command us to do?' He said: 'Sacrifice, whatever month it is, do good for the sake of Allah, the Mighty and Sublime, and feed (the poor).' He said: 'We used to sacrifice the Fara' during the Jahiliyyah; what do you command us to do?' He said: 'For every flock of grazing animals, feed the firstborn as you feed the rest of your flock until it reaches an age where it could be used to carry loads, then sacrifice it, and give its meat in charity, for that is good."'