Musannaf Ibn Abi Shayba 7:282

'Umar ibn Ayyub narrated to us, from 'Umar ibn Zayd, who said: Two men came to al-Qasim and asked him—and I was listening—about a man who made it incumbent upon himself to walk to the House of Allah. He said: Al-Qasim said, "He has made a vow." He said: "Then let him offer an expiation for his oath."