I heard the Messenger of Allah ﷺ, and had I wished to kiss the seal between his shoulders from how close I was to him, I could have done so, say, "The Throne of the Most Merciful shook," referring to Saad bin Muaz on the day he died.
سمعت رسول الله ﷺ- لو أشاء أن أقبل الخاتم الذي بين كتفيه من قربي من قربي منه لفعلت- يقول: اهتز عرش الرحمن تبارك وتعالى. يريد سعد بن معاذ يوم توفي.
Imam Ahmad (26793), Tirmidhi in Shamail (17), and Tabarani in Al-Kabir (24/267) all narrated it through various chains going back to Yusuf bin Yaqub Majishun, from his father, from Asim bin Umar bin Qatada, from his grandmother Rumaytha, who related the same account. Its chain is hasan because of Yusuf's father, Yaqub bin Abu Salama Majishun Taymi, their freedman, who is truthful. As for the shaking of the Throne at the death of Saad bin Muaz, see the chapters on the Throne.As for what was reported from Ibn Umar, that he said, "The seal of prophethood on the back of the Messenger of Allah ﷺ was like a lump of flesh, on which was written, Muhammad is the Messenger of Allah," this is daif. Ibn Hibban narrated it in his Sahih (6302) through Raja bin Murajja, who said that Ishaq bin Ibrahim, the judge of Samarqand, narrated to them, that Ibn Jurayj narrated to them from Ataa, from Ibn Umar, and he related it.Ishaq bin Ibrahim is Abu Ali Samarqandi the judge. Ibn Hibban mentioned him in his Thiqat (109/8), noting that two people narrated from him, one of them being Raja, and I have not found anyone else's confirmation of his reliability. So he is acceptable according to the terminology of Ibn Hajar, except that he lacks corroboration, which makes his hadith somewhat weak. As for the statement "written on it, Muhammad is the Messenger of Allah," it is rejected, and this has not been established in any sahih hadith.Ibn Hajar said in Al-Fath (563/6), "As for what has been reported, that it was like the mark of a cupping instrument, or like a black or green mole, or that it had written on it, Muhammad is the Messenger of Allah, or, march forth, for you are victorious, or something similar, none of that has been established." Then he said, "Do not be deceived by what occurs regarding this in Sahih Ibn Hibban, for he was inattentive when he graded it sahih, and Allah knows best."Hafiz Haythami said in Mawarid Zaman (2097), "Some narrators confused the seal of prophethood with the seal he used to seal letters with." Its editor, Shaykh Muhammad Abdul-Razzaq Hamza, noted that in the margin of the original, written in the hand of Shaykh al-Islam Ibn Hajar, may Allah have mercy on him, it says, "The one referred to as 'some' is Ishaq, and he is daif."The rest of the hadiths on this subject can be found in the section on the virtues of the Prophet ﷺ and his fragrant biography.