Since standing, recitation, bowing and prostrating fully, purification with water, and other such duties are waived when one is unable to perform them, so too are standing in a row and refraining from standing ahead of the imam. The same principle extends to the remaining questions concerning prayer rows, such as praying without seeing either the imam or those behind him while still hearing the declarations of “Allah is the Greatest,” and other such cases. The imam is placed ahead so that those praying behind him can see and follow him; this consideration does not apply to a follower.
As for the hadith of Abū Bakrah, it does not state that he prayed alone behind the row before the imam raised his head from bowing. He thus fulfilled enough of the prescribed requirement to join the row for the unit of prayer to count. His case is like that of someone who stands alone and is then joined by another person standing beside him during the standing portion of the prayer. This is permissible by agreement of the imams. The hadith of Abū Bakrah contains a prohibition in the Prophet’s words:
“Do not do that again.”
It does not state that he ordered him to repeat that unit of prayer, unlike the hadith concerning the man praying alone, whom he ordered to repeat the prayer. The latter report is clear and explicit, whereas the former leaves the matter unspecified. Even supposing that the hadith of Abū Bakrah expressly stated that he joined the row after the imam had stood upright—as is permitted according to one of the two views in Ahmad’s school and by others—this would be permissible in such a case, but not in the case in which he ordered the prayer to be repeated. Each of these views has a basis.
As for distinguishing between someone who knows the prohibition and someone who does not, as one view within Ahmad’s school does, this is untenable. The man who prayed alone did not know of the prohibition, yet the Prophet ordered him to repeat the prayer, just as he ordered the Bedouin who prayed incorrectly to repeat his prayer.