Nasai

(1) "He swayed"—that is, out of joy at his arrival. This meaning indicates their greatness and status.

(2) "He was compressed" because every human being has some deficiency (except for the Prophets, alayhimussalam, for they are infallible). Through this compression, he is freed from the effect of that deficiency, provided he is a believer. A believer is compressed only once, then released, but it is not unlikely that for a disbeliever this torment occurs repeatedly. It is mentioned in the hadith that the grave compresses everyone; if anyone were to be spared from it, surely Sa'd ibn Mu'adh radi Allahu anhu would have been spared. (Al-Mawsu‘ah al-Hadithiyyah, Musnad al-Imam Ahmad: 327/40, no. 24283, and al-Sahihah: 268/4, no. 1695)

(3) In explanation of this, it has also been said that the "grave" is like a mother to a person, because he was created from that very soil. When a son meets his mother after a long time, she embraces him tightly to her body, even if it causes him discomfort. The matter of the grave is similar; however, it embraces the righteous person with love, and the wicked person with anger and displeasure. For the righteous, there is delight in it, and for the wicked, punishment. And Allah knows best.

Hadith Authentication:
The chain of narration (isnad) of this hadith is authentic.
Besides Sunan an-Nasa’i, this narration is also mentioned in Dala’il an-Nubuwwah by al-Bayhaqi [28/4].

Jurisprudence of the Hadith:
➊ The jolt (shock) of the grave is a reality for every deceased person.
➋ Hafiz Dhahabi states:
«هٰذا الضمة ليست من عذاب القبر فى شي بل هو أمر يجده المؤمن كما يجد ألم فقد ولده وحميمه فى الدنيا وكما يجد ألم مرضه وألم خروج روحه . . .»
“This jolt (for the believer) is not from the punishment of the grave, rather it is like the grief a believer feels at the loss of his child or something beloved, and like the pain of illness and the agony of the soul’s departure...” [سیر اعلام النبلاء 1؍290]
◄ Then Hafiz Dhahabi says:
“We know that Sa’d (radi Allahu anhu) is among the people of Paradise and among the most eminent of martyrs.” [ایضاً ص 290]
➌ In this hadith, there is a tremendous virtue of our master Sa’d ibn Mu’adh (radi Allahu anhu): that Allah’s Throne moved with love and affection at his martyrdom, and seventy thousand angels attended his funeral prayer. «سبحان الله»
➍ There are many gates of the heavens, which only Allah knows.
➎ In one narration, it is mentioned that our master Sa’d (radi Allahu anhu) did not take care to avoid drops of urine.
This narration is weak and rejected due to an unknown narrator and being disconnected (munqati‘).
For other weak and rejected narrations in this regard, see: [مرعاة المفاتيح ج1 ص232]