Musnad Ahmad 5:342

Mansur ibn Wardan al-Asadi narrated to us, Ali ibn Abd al-A’la narrated to us, from his father, from Abu al-Bakhtari, from Ali, who said: When this verse was revealed: "And [due] to Allah from the people is a pilgrimage to the House - for whoever is able to find thereto a way" [Al Imran: 97], they said: "O Messenger of Allah, is it every year?" He remained silent. They said: "Is it every year?" He remained silent. He said: Then they said: "Is it every year?" He said: "No. And if I had said 'yes,' it would have become obligatory." Then Allah the Exalted revealed: "O you who have believed, do not ask about things which, if they are shown to you, will distress you" [Al-Ma'idah: 101], to the end of the verse.