Musannaf Ibn Abi Shayba 7:271

Ibn Numayr narrated to us, he said: Isma'il bin Abi Khalid narrated to us, from al-Sha'bi, that he was asked about a man who vowed to walk to the Ka'bah, so he walked half the distance and rode the other half. He said: Ibn Abbas said, "He shall ride what he walked, and walk what he rode in the following year, or he shall offer a sacrificial camel."