Mustadrak al-Hakim — Hadith 1805

It is narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that on the occasion of the Treaty of Hudaybiyah the people were commanded to sacrifice replacements for the sacrificial animals (that had been held back) in the year they would enter Makkah. So they replaced them, and since camels had become expensive and scarce, they were granted the concession that whoever could not find a camel could buy a cow and sacrifice it.