Case 1: if a two-rakah nafl is begun and invalidated, the Hanafi makeup is two rakahs.
Case 2: in a four-rakah nafl where the first pair has been completed with its sitting and the later pair is invalidated, Abu Hanifa and Muhammad require two-rakah makeup, while Abu Yusuf requires four.
The distinction turns on whether the first pair reached completion.
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Voluntary Prayers: Regular Sunnahs, Nawafil and Juristic Differences
Case 1: if a two-rakah nafl is begun and invalidated, the Hanafi makeup is two rakahs.
Case 2: in a four-rakah nafl where the first pair has been completed with its sitting and the later pair is invalidated, Abu Hanifa and Muhammad require two-rakah makeup, while Abu Yusuf requires four.
The distinction turns on whether the first pair reached completion.