Yahya related to me from Malik that Abdullah ibn Abi Habiba said,"I said to a man, when I was young, 'A man who only says that he mustwalk to the House of Allah and does not say that he has vowed to walk,does not have to walk.' A man said, 'Shall I give you this smallcucumber?' and he had a small cucumber in his hand and you will say,'I must walk to the house of Allah?' I said, 'Yes' and I said it, forat that time I was still immature. Then, when I came of age, some onesaid to me that I had to fulfill my vow. I went and asked Said ibn al-Musayyab about it, and he said to me, 'You must walk.' So I walked."Malik said, "That is the custom among us."
USC-MSA web (English) reference: Book 22, Hadith 3
Arabic reference: Book 22, Hadith 1014