Hadith: The Messenger of Allah (may Allah’s peace and blessings be upon him) saw a man pray alone behind the first row, and he commanded him to repeat the prayer

عن وابِصَة بن مَعْبَد الجُهني رضي الله عنه أن رسول الله صلى الله عليه وسلم رأى رَجُلا يصلِّي خلف الصَّف وحْدَه، فأمَرَه أن يُعِيد الصلاة.
[حسن] - [رواه أبو داود وأحمد والترمذي]

Wābisah ibn Ma‘bad al-Juhani (may Allah be pleased with him) reported: The Messenger of Allah (may Allah’s peace and blessings be upon him) saw a man pray alone behind the first row, and he commanded him to repeat the prayer.
Hasan/Sound. - [At-Tirmidhi]

Explanation

As the Messenger of Allah (may Allah’s peace and blessings be upon him) finished his prayer and was leaving the mosque, he saw a man pray alone behind the first row. He ordered him to repeat his prayer. The order was given in explicit terms, unequivocally indicating that the prayer offered by a person alone behind the row of congregational prayer is not valid. Indeed, the Prophet's order to this man to repeat his prayer would not have been given with regard to something merely recommended. What is reported in another Hadīth, that Abu Bakrah (may Allah be pleased with him) bowed down outside the row and subsequently joined it, does not actually contradict the ruling in question, for Abu Bakrah did not pray alone. That he made the Takbīr of Ihrām and performed part of the Rukū‘ alone does not mean that he offered the prayer alone, unlike a person who offers a whole Rak‘ah or more alone, regardless of whether the row is complete or not. Hence, if a person finds some room in the row into which he can fit himself, it is not permissible for him to stand alone behind the row; and if he did do so, his prayer is invalid. However, if he does not find enough space in the row, he can stand alone behind it, but he should not leave the congregation.