Ibn 'Umar (may Allah be pleased with him) narrated: That ‘Umar said
Apostle! I vowed to perform I'tikaf for one night in Al-Masjid-al-Haram, during the Pre-Islamic Period of ignorance
(before embracing Islam).
"The Prophet said, "Fulfill
your vow." (Bukhaari)
Al-Bara (may Allah be pleased with him) narrated: That the Prophet ordered us to help others to fulfill the vows/oaths. (Bukhaari)
(2) If one makes a vow before becoming a Muslim,
he must still
fulfil this vow.