Muslim

Hadith Commentary: Benefits and Issues: (وَكَانَت صَلَاتُهُ بَعْدُ تَخْفِيفًا)
The scholars have mentioned various meanings for this phrase.

➊
Despite reciting Surah Qaf, the prayer of the Prophet (sallallahu alayhi wa sallam) was light, so he maintained this lightness, and Jabir bin Samurah (radi Allahu anhu) in the next narration considers the recitation of Surah Qaf itself as lightness.

➋
The prayers after Fajr, that is, Dhuhr, Asr, Maghrib, and Isha, all were lighter compared to Fajr, and in these, the Prophet (sallallahu alayhi wa sallam) would recite less compared to Fajr.

➌
In the early period, when the number of the Companions (radi Allahu anhum ajma'in) was small and those praying behind the Prophet (sallallahu alayhi wa sallam) were “the foremost, the first” [], who were at the highest level in faith and deeds, his prayers were generally lengthy. In the later period, when the number of those praying with him increased, and they included traders or farmers, and among them were also those who were lesser in faith and deeds compared to the former ones, and due to the increased number of worshippers, the number of sick, weak, and elderly also increased, so the Prophet (sallallahu alayhi wa sallam) began to make the prayer lighter compared to before.

➍
He would always recite Surah Qaf in the first rak‘ah, as Ziyad bin ‘Ilaqah narrated from his uncle, and in the second rak‘ah, the Prophet (sallallahu alayhi wa sallam) would lighten it. It was the blessed habit of the Prophet (sallallahu alayhi wa sallam) that he would make the first rak‘ah long.