What is the ruling on someone who vowed to perform I'tikaf (spiritual retreat) for ten days in Makkah, but only performed it for five days and was unable to complete it? Is he obligated to complete the duration, or is an expiation (kaffarah) required of him?

Whoever makes a vow, such as an i'tikaf (spiritual retreat), is obligated to fulfill it, based on the saying of the Messenger of Allah (peace and blessings be upon him): "Whoever vows to obey Allah, let him obey Him." And according to Ibn Qudamah: "Undertaking an act of obedience in exchange for a blessing sought or a tribulation averted, such as his saying: 'If Allah cures me, then I owe Allah a month of fasting...' This must be fulfilled by the consensus of the people of knowledge."

Therefore, you must make up the remaining five days. If you are unable to fulfill it, then you must offer an expiation (kaffarah) for breaking an oath, which is feeding ten poor people or clothing them. If you are unable to do that, then fasting three days.