It is narrated from Mujahid (may Allah have mercy on him) that al-Harith bin Suwayd first accepted Islam, then he (apostatised and) went back to his people and adopted disbelief; so Allah Almighty revealed this blessed verse: "How shall Allah guide a people who disbelieved after their belief and had testified that the Messenger is true" [Surah Al Imran: 86].
The narrator says that a man from al-Harith's people conveyed these verses to him and recited them to him, and al-Harith (on hearing these verses) said: "By Allah! As far as I know, you are certainly truthful, and the Messenger of Allah (peace and blessings be upon him) is even more truthful than you, and Allah Almighty is the most truthful of the three." Then he came back and accepted Islam again with great excellence and sincerity.
Hadith Referenceالمطالب العالية بزوائد المسانيد الثمانية / كتاب التفسير / 3561
Hadith Gradingمحدثین:إسناده حسن، مرسل
Hadith Takhrij«حکم الحدیث المطالب العالیہ محققاً تنسیق د. سعد بن ناصر بن عبد العزيز الشَّثري- مکتبہ شاملہ :هذا إسناد مرسل حسن الإِسناد إلى مجاهد » «انفرد به المصنف من هذا الطريق»