What is the poetry that Qutaybah bint Al-Harith wrote to the Messenger of Allah, peace and blessings be upon him, which made the Messenger say: "By Allah, if her poetry had reached me before I killed him, I would not have killed him"?

The poetry belongs to Qutaylah bint Al-Nadr ibn Al-Harith, not Qutaybah, and it is as follows:

O rider, if Al-Atheel is a place of conjecture, From the morning of the fifth day, and you are successful, Convey to a deceased one greetings, That steeds will not cease to flutter with them. From me to you, and a flowing tear, Its streams have poured forth, and another is choked. Will Al-Nadr hear me if I call him? Or how can a dead person who does not speak hear? O Muhammad, O best offspring of a noble woman, Among her people, and the sire is a purebred stallion. What harm would it have caused you if you had shown grace? And perhaps, A youth might show grace while he is angry and enraged. Or if you would accept a ransom, then let him spend, The most precious of what can be spent for ransom. For Al-Nadr is the closest of those taken captive in kinship, And the most deserving of being freed, if freedom is granted. The swords of his father's tribe continued to strike him; For the sake of Allah, there are kinships that are torn apart. Patiently he was led to Madinah, wearied, Shackled in his steps, bound and tied.

Ibn Hisham mentioned that when this poetry reached the Prophet, peace and blessings be upon him, he said: "Had this reached me before his killing, I would have shown him grace."