Book 22, Number 22.2.5:
Yahya related to me from Malik that Yahya ibn Said said, "I vowed to walk, but I was struck by a pain in the kidney, so I rode until I came to Makka. I questioned Ata ibn Abi Rabah and others, and they said, 'You must sacrifice an animal.' When I came to Madina I questioned the ulama there, and they ordered me to walk again from the place from which I was unable to go on. So I walked."
Yahya said that he had heard Malik say, "What is done among us regarding someone who makes a vow to walk to the House of Allah, and then cannot do it and so rides, is that he must return and walk from the place from which he was unable to go on. If he cannot walk, he should walk what he can and then ride, and he must sacrifice a camel, a cow, or a sheep if that is all that he can find."
Malik, when asked about a man who said to another, "I will carry you to the House of Allah", answered, "If he intended to carry him on his shoulder, by that he meant hardship and exhaustion to himself, and he does not have to do that. Let him walk by foot and make sacrifice. If he did not intend anything, let him do hajj and ride, and take the man on hajj with him. That is because he said, 'I will carry you to the house of Allah.' If the man refuses to do hajj with him, then there is nothing against him, and what is demanded of him is cancelled."
Yahya said that Malik was asked whether it was enough for a man who had made a vow that he would walk to the House of Allah a certain (large) number of times, or who had forbidden himself from talking to his father and brother, if he did not fulfil a certain vow, and he had taken upon himself, by the oath, something which he was incapable of fulfilling in his lifetime, even though he were to try every year, to fulfil only one or a (smaller) number of vows by Allah? Malik said, "The only satisfaction for that that I know is fulfilling what he has obliged himself to do. Let him walk for as long as he is able and draw near Allah the Exalted by what he can of good."