Is it valid for a person praying alone (Munfarid) to have someone join them later and form a congregation by intending to follow the initial solo worshipper in a supererogatory (Nafl) prayer?

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This is valid in supererogatory prayer. Imam Ahmad explicitly affirmed this. The justification rests upon the Hadith of Ibn Abbas where the Prophet (peace be upon him) prayed voluntarily at night, and Ibn Abbas joined him from the left side, whereupon the Prophet took his hand and moved him to the right side. This Hadith is agreed upon (Muttafaqun 'alayh), with the version provided being that of Muslim.