Musnad Abu Ya'la al-Mawsili 3:138

Zuhayr Abu Khaythama narrated to us, Ibn 'Uyayna narrated to us, from 'Amr, that al-Hasan ibn Muhammad informed him that 'Ubayd Allah ibn Abi Rafi' informed him that 'Ali said: The Messenger of Allah (ﷺ) sent me, al-Miqdad, and al-Zubayr to Rawdat Khakh. He said: "There is a woman there with a letter." He said: So we went out, our horses racing with us, and we arrived to find the woman. We said: "You will either bring out the letter, or we will search your clothes." He said: She brought out a letter from her hair braid. It contained [a message] from Hatib ibn Abi Balta'a to the people of Makkah, informing them of some of the affairs of the Messenger of Allah (ﷺ). The Prophet (ﷺ) said: "What is this, O Hatib?" He said: "O Messenger of Allah, I did not write it out of apostasy from my religion," and he offered an excuse, the meaning of which was that he was a stranger among them, or something similar. 'Umar said: "O Messenger of Allah, let me strike the neck of this hypocrite." He said: "And what do you know, O 'Umar? Perhaps Allah has looked upon the people of Badr and said: 'Do what you wish, for I have forgiven you.'"