Anas (may Allah be pleased with him) said: An old man passed by, supported by his two sons. He (peace and blessings be upon him) said: "What is the matter with him?" The reply came: He has vowed to perform Hajj on foot. He (peace and blessings be upon him) said: "Allah has no need of this man tormenting himself; let him ride." So he rode.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النذور / 20109