The Udhiyah (sacrifice) is one of the religious symbols (Shaa'er) of Muslims in Eid Al Adha, and its legislation came from the following hadith reported by Ahmad and IbnMajahon the authority AbiHuraira(may Allah be pleased with him) who said that the prophet (PBUH) said:

((Let those who can afford slaughtering a sacrifice, yet they don't, never come near our Musallah (place of prayer).))

[Ahmad]

 Abu Hanifah, May Allah have mercy on his soul, deduced from this Hadith that the Udhiyah is an obligation, as this kind of threat can’t be applied on an act that is not an obligatory one.
Whereas other than Ahnaf said: it is an evidence-based Sunnah, and they have their proofs for it.
Udhiyah is mandadory once every year by the free, adult, prudent, settled and rich Muslim.
Aishah, May Allah be pleased with her narrated that the prophet PBUH said:

((No act performed on the Day of Nahr by son of Adam is more beloved to Allah than shedding the blood (slaughtering the Udhiyah). Verily, it comes on the Day of Judgement with its horns, hairs, and nails. Allah gives reward for it before its blood reaches the ground, so be content for sacrificing Udhiyah.))

[Al-Tirmidhi and IbnMajah]

  Anas also narrated that:

((the Prophet (peace be upon him) sacrificed two horned rams which were white. He said: and I saw him slaughtering them with his own hands, and I saw him putting his foot on them while mentioning the Name of Allah and glorifying Him (saying Allah is the Greatest). ))