What is the ruling for a worshipper joining the Maghrib prayer (Isha) in the third rak'ah?
Chapter on the Times When Prayer is Prohibited
Al-Mughni
Book of Prayer
Primary text
If a worshipper joins the Maghrib prayer when the Imam is in the third rak'ah (meaning they join for the final odd unit, making their first unit the third rak'ah of the Imam), they must complete their prayer with an even number of rak'ahs, specifically two rak'ahs to pair with the one they joined. This view is explicitly stated by Ahmad, and it is the position of Al-Aswad bin Yazid, Al-Zuhri, Al-Shafi'i, and Ishaq. It is narrated that Qatadah narrated from Sa'id bin Al-Musayyib holding this view. The justification is that the prayer being performed is a supererogatory act (nafl). It is not legislated to perform an odd unit (witr) as a voluntary prayer except for the Witr prayer itself. Therefore, adding an extra rak'ah (making it two remaining units for the follower instead of one) is preferable to subtracting a unit, lest the follower separates from the Imam before completing the obligatory nature of the prayer structure.
Supporting text
There is a narration from Silah, from Hudhayfah, that when he missed the Maghrib prayer and completed it, he said: 'I stood up for the third rak'ah, but he made me sit down.' This narration is subject to two interpretations: either it implies an instruction to limit the completion to two rak'ahs to ensure the first part is an even number (shaf'), or it implies an instruction to pray exactly as the Imam prayed.