What is the authenticity of the hadith: "Pray the prayer of the most moderate among you—or the weakest among you"?

It was not narrated with this exact wording. However, the Imam lightening the prayer is a prescribed matter to accommodate the weak, according to the hadith: "Whoever leads a people in prayer should lighten it, for among them are the elderly, the sick, the weak, and those with needs." And the hadith: "You are their Imam, so emulate their weakest." The meaning of emulating their weakest is to lighten the recitation and glorifications without abandoning the pillars [of prayer].