Praying alone behind a row and indicating to someone in an earlier row to stand with the one standing behind the rows
Is it valid for a person to stand alone in prayer behind a row due to lack of space in the previous row? Is it permissible for him in such a case to indicate to someone in the previous row to stand with him by drawing him back?
Praying alone occurs either in an absolute sense or in a restricted sense. When it occurs in its absolute sense, it is different than group prayer as indicated by the words of the Prophet [pbuh] who said: "Congregational prayer is superior to praying alone by twenty seven degrees" [Bukhari and Muslim]. If it occurs in its restricted sense as mentioned in the question, it is referred to as 'praying alone behind a row'.
Praying alone behind a row
It is valid for a person to stand alone behind a row if he has an excuse such as not finding another person to stand with him, but disliked when there is no excuse. This is attested to by the hadith narrated by Abu Bakra, may Allah be pleased with him, who said that he once joined prayers when the Prophet [pbuh] was making ruku' [prostration]. So he likewise made ruku' before reaching the row. Later, he mentioned this to the Prophet [pbuh] who said: "May Allah increase your devotion, but do not repeat this." From this hadith, scholars deduced that it is unnecessary for a person who stands alone in a row in group prayer to repeat his prayer. They further concluded that what was mentioned in the hadith narrated by Wabsa ibn Mu'bad, may Allah be pleased with him, and compiled by al-Tirmidhi, that the Prophet [pbuh] ordered a person to repeat his prayer when he saw him praying alone behind a row is only by way of recommendation (not an obligation), thus establishing agreement between the two hadiths.
Basing their opinion on Wabsa's hadith which they interpreted as indicating obligation, the Hanbalis consider the prayer of a person who prays one complete rak'a (cycle of prayer) while standing alone behind a row without an excuse as invalid.
1- The Malikis maintained, as did the Shafi'is in one of their opinions, that a person who joins prayer and does not find a gap or space in the row is to stand alone and refrain from drawing someone from the previous row. This is because he will deprive the person he draws back of the virtue of standing in an earlier row. This is the opinion which al-Bayuti mentions in his book, citing al-Shafi'i's opinion as well as the opinion chosen for fatwa by al-Qadi al-Tayyeb. The Malikis added that the person selected should not comply. This is likewise the opinion of Kamal ibn al-Humam from among the Hanafis.
2- The opinion of the Hanafis and the preponderant opinion of the Shafi'is is that it is desirable for a person who is standing alone behind the row to indicate to someone in an earlier row to stand with him, but only when he expects that the person selected will oblige him. Otherwise, he is not to draw him back to avoid creating a commotion.
3- According to the Hanbalis, such a person is to stand to the right of the imam if possible, since this is the place he would occupy if he were to pray alone behind the imam. If this is not possible, he may indicate to someone in an earlier row to stand with him, or else pray alone behind the row; it is disliked to indicate to someone to stand with him by drawing him back. Ahmed and Ishaq found this offensive since it does not entail asking for that person's permission.
According to the majority of scholars, the prayer of a person standing alone behind the row is valid provided he cannot find a gap or space in which to stand in an earlier row. Those who permit drawing someone back from an earlier row, stipulate his consent in advance. For this reason, we maintain that permissibility is contingent upon that person's consent. If the person who is praying alone behind the row does not know whether or not the selected person will approve, he is not to draw him back out of respect and to avoid creating a commotion.
Allah Almighty knows best.