Heraclius's Test of the Three Signs (Hadith of al-Tanukhi)

This is the second letter to pass between the Prophet ﷺ and Heraclius, and unlike the first it is not in al-Bukhari but in the Musnad of Imam Ahmad, told by the Tanukhi himself, a Christian Arab of the tribe of Tanukh whose name no one recorded, to a young man of Hims who found him in his old age, nearly a hundred, and asked him about the letters. When the Prophet ﷺ was at Tabuk he sent Dihyah al-Kalbi to Heraclius again. The emperor, who had watched what this man had done since the first letter, locked himself in with his patriarchs and priests and put the invitation to them, and then, needing an Arabic speaker with a good memory, was brought the Tanukhi. He gave him a letter and three things to check: whether the Prophet ﷺ spoke of the letters he had sent to the kings, whether he answered a question in the letter with the word night, and whether he had a mark on his back.

The Tanukhi found him sitting with his Companions by the well at Tabuk, walked up and handed him the letter. The Prophet ﷺ set it aside and asked where he was from, and invited him to Islam, the hanifiyyah, the religion of his father Ibrahim. The Tanukhi answered that he was an ambassador and would not change his people's religion while he carried their message; he would go home and think. The Prophet ﷺ laughed and recited that you do not guide whom you love, but Allah guides whom He wills; and then said that he had written to Kisra of Persia, who tore his letter, so Allah would tear his kingdom, to the Najashi, who tore it, so Allah would tear his, and to the Tanukhi's master, who had kept it safe, so that men would go on fearing his power as long as there was good in life. That was the first sign, and the envoy took an arrow from his quiver and scratched a mark on his parchment. He asked who read for the Prophet ﷺ, and Mu'awiyah was called and read Caesar's question: you invite me to a Garden as wide as the heavens and the earth, so where is the Fire? Subhan Allah, the Prophet ﷺ answered, where does the night go when the day comes? The second mark went on the parchment. Then the Prophet ﷺ said that the envoy had a right to a gift and there was nothing to give him on campaign; Uthman stood and presented a yellow robe, and an Ansari took him home for the night.

He waited a day or two for a chance at the third sign and gave up, and went to take his leave. The Prophet ﷺ told him to wait, turned, let his shirt down from his shoulders and said: go and tell your master what you have seen. The Tanukhi described the Seal of Prophethood between his shoulder blades as like a large cupping mark. He went home with all three signs checked, and some say he later became a Muslim and some that he died a Christian. What Heraclius did with the answer is told in Bukhari's second story of him: he put the question to his senators and, when they recoiled, said he had only been testing them.