Hilal bin Yasaf says: In an island, Ziyad bin Abi Al-Ja’d showed me an old man, he was called Wabisah bin Ma’bad. He made me stand by this elder and said: This person has narrated to me this hadith that the Messenger of Allah (peace and blessings be upon him) saw a man praying alone in the row and ordered him to repeat the prayer, so he prayed again. Abdullah says: My father (Imam Ahmad bin Hanbal) also held the same opinion as mentioned in this hadith.
Hadith Referenceالفتح الربانی / أبواب صلاة الجماعة / 2669