Hadith: O Messenger of Allah, I had vowed, before embracing Islam, to make I‘tikāf for one night – another narration reads: for one day – the Sacred Mosque. He said: Then fulfill your vow.

عن عمر بن الخطاب رضي الله عنه قال: قلت: يا رسول الله، إني كنت نَذَرْتُ في الجَاهلية أن أعتكف ليلة -وفي رواية: يومًا- في المسجد الحرام ؟ قال: «فَأَوْفِ بِنَذْرِكَ».
[صحيح] - [متفق عليه]

‘Umar ibn al-Khattāb (may Allah be pleased with him) reported: "I said: "O Messenger of Allah, I had vowed, before embracing Islam, to make I‘tikāf for one night – another narration reads: for one day – the Sacred Mosque." He said: "Then fulfill your vow."
[Authentic hadith] - [Narrated by Bukhari & Muslim]

Explanation

‘Umar ibn al-Khattāb (may Allah be pleased with him) vowed in the pre-Islamic era that he would seclude himself for worship one night in the Sacred Mosque. So he asked the Prophet about the ruling regarding his vow, which he had made before embracing Islam. The Prophet (may Allah's peace and blessings be upon him) ordered him to fulfill his vow.