Musnad Ahmad Hadith 20723

It is narrated from Nubaisha (may Allah be pleased with him) that once people asked the Prophet (ﷺ), "O Messenger of Allah, in the days of ignorance we used to offer sacrifices in the month of Rajab. What command do you give us regarding this?" The Prophet (ﷺ) said, "You may slaughter in the name of Allah in whichever month you wish. Do good deeds for Allah and feed the people." The companions asked, "O Messenger of Allah, in the days of ignorance we used to also slaughter the firstborn of the animal. What command do you give us regarding this?" The Prophet (ﷺ) said, "Every grazing animal has a firstborn which you feed and water. When it becomes capable of bearing a load, then you may slaughter it and give its meat in charity. He probably said this for the traveler, that this is better for you." And the Prophet (ﷺ) said, "We had forbidden you from eating the meat of sacrifice for more than three days so that it would reach all of you. Now Allah has granted abundance, so eat it, store it, and trade with it. And remember, the days of Tashreeq are days of eating, drinking, and remembering Allah."