Is the prayer of a solitary person behind the Imam valid if there is no one else with him?

If there is only one male follower (ma'mum), the Sunnah is for him to stand to the right of the Imam. As for a person praying alone behind the row (monfarid), scholars have differed on this matter, holding three opinions: invalidity absolutely (the Hanbali madhhab), validity with dislike (the majority's madhhab), and validity only with an excuse (chosen by Sheikh al-Islam Ibn Taymiyyah). The apparent meaning of the Ahadith indicates that both the ignorant and the knowledgeable are equal in this regard. Whoever prays in an incorrect manner, being ignorant of the ruling, is commanded to repeat the prayer if its time is still remaining. If its time has elapsed, he is not obligated to repeat it, but he must correct his prayer in the future. Therefore, the questioner is not obligated to repeat the previous prayer, but he should not repeat this action.