His statement, “seated,” is a circumstantial qualifier referring to the subject of “they pray.”
His statement, “as a recommendation,” means that this ruling is recommended, not obligatory. Recommendation is sunnah; that is, it is sunnah for them to pray seated behind him.
The evidence for this is the Prophet’s saying: “The imam has only been appointed to be followed,” until he said: “When he prays standing, then pray standing; and when he prays seated, then pray seated, all of you.” (1) This is an explicit text showing that the prayer of one able to stand behind one unable to stand is valid, and that he prays seated behind his imam, following his imam.
His statement, “and they pray seated behind him as a recommendation,” informs us—may Allah have mercy on him—that if they were to pray standing behind him, their prayer would be valid, because omitting a sunnah does not invalidate the prayer.
Some scholars held that prayer behind him must be performed seated.
They cited the following as evidence:
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The Messenger’s saying: “Pray seated.” The basic implication of a command is that it is obligatory, especially since the Prophet ﷺ explained this at the beginning of the hadith by saying: “The imam has only been appointed to be followed.”
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On one occasion, when he ﷺ led his Companions in prayer while unable to stand, they stood, so he gestured to them to sit.