Expedition of Khalid ibn al-Walid (Najran)
The event is mentioned partly in the Quran verse 3:61. The event is also mentioned by the Muslim jurist Tabari, who mentions that Muhammad wrote a letter to Khalid ibn al-Walid. Those letters are also mentioned by Muhammad Hamidullah in his French book Six Originaux des letters diplomatiques du Prophete e l'Islam, which contains a collection of six of the Muhammad's letters whose original texts have been preserved, that he compiled. Muhammad Hamidullah considered the letters authentic. The translator of Tabari, Volume 9, The last Years of the Prophet, Isma'il Qurban Husayn, mentions in the footnote on page 83, that the letter mentioned by Tabari is also in the texts collected by Muhammad Hamidullah.