What is the minimum requirement for an absolute vow of prayer (Salat)?

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There are two narrations regarding the minimum for an absolute vow of prayer. One states that one rak'ah suffices, based on the fact that the minimum of prayer is one rak'ah (as exemplified by Witr prayer) and that 'Umar (may Allah be pleased with him) voluntarily prayed a single rak'ah. The second narration states that two rak'ahs are the minimum, which is the view held by Abu Hanifa. This is because the minimum prayer established by divine decree is two rak'ahs, so the vow must be interpreted as such, as Witr is considered voluntary (nafl) while the minimum obligatory prayer is more fundamental.