I saw the seal on his back as if it were a pigeon's egg.  

رَأَيْتُ خَاتِمًا فِي ظَهْرِ رَسُولِ اللَّهِ ﷺ كَأَنَّهُ بَيْضَةُ حَمَامٍ  

This hadith has been narrated on the authority of Simak with the same chain of transmitters.  

عَنْ سِمَاكٍ بِهَذَا الإِسْنَادِ مِثْلَهُ  

My mother's sister took me to Messenger of Allah ﷺ and said: Messenger of Allah, here is the son of my sister and he is ailing. He touched my head and invoked blessings upon me. He then performed ablution and I drank the water left from his ablution; then I stood behind him and I saw the seal between his shoulders.  

سَمِعْتُ السَّائِبَ بْنَ يَزِيدَ يَقُولُ ذَهَبَتْ بِي خَالَتِي إِلَى رَسُولِ اللَّهِ ﷺ فَقَالَتْ يَا رَسُولَ اللَّهِ إِنَّ ابْنَ أُخْتِي وَجِعٌ فَمَسَحَ رَأْسِي وَدَعَا لِي بِالْبَرَكَةِ ثُمَّ تَوَضَّأَ فَشَرِبْتُ مِنْ وَضُوئِهِ ثُمَّ قُمْتُ خَلْفَ ظَهْرِهِ فَنَظَرْتُ إِلَى خَاتِمِهِ بَيْنَ كَتِفَيْهِ مِثْلَ زِرِّ الْحَجَلَةِ  

I saw Allah's Apostle ﷺ and ate with him bread and meat, or he said Tharid (bread soaked in soup). I said to him: Did Allah's Apostle ﷺ seek forgiveness for you? He said: Yes, and for you, and he then recited this verse:" Ask forgiveness for thy sin and for the believing men and believing women" (xlvii. 19). I then went after him and saw the Seal of Prophethood between his shoulders on the left side of his shoulder having spots on it like moles.  

رَأَيْتُ النَّبِيَّ ﷺ وَأَكَلْتُ مَعَهُ خُبْزًا وَلَحْمًا أَوْ قَالَ ثَرِيدًا قَالَ فَقُلْتُ لَهُ أَسْتَغْفَرَ لَكَ النَّبِيُّ ﷺ قَالَ نَعَمْ وَلَكَ ثُمَّ تَلاَ هَذِهِ الآيَةَ { وَاسْتَغْفِرْ لِذَنْبِكَ وَلِلْمُؤْمِنِينَ وَالْمُؤْمِنَاتِ} قَالَ ثُمَّ دُرْتُ خَلْفَهُ فَنَظَرْتُ إِلَى خَاتَمِ النُّبُوَّةِ بَيْنَ كَتِفَيْهِ عِنْدَ نَاغِضِ كَتِفِهِ الْيُسْرَى جُمْعًا عَلَيْهِ خِيلاَنٌ كَأَمْثَالِ الثَّآلِيلِ