Sayyiduna Ibn Abbas (may Allah be pleased with them both) says that Sayyiduna al-Aqra' bin Habis (may Allah be pleased with him) said: O Messenger of Allah (peace and blessings be upon him)! Is Hajj every year? He said: "No; rather, whoever performs another Hajj after the one Hajj, it is voluntary." And he said: "Had I said yes, it would have become obligatory, and had it become obligatory, you would not have been able to listen and obey."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9847