Abdullah bin Abbas reported,

When news of the Prophet's ﷺ mission reached Abu Dharr, he said to his brother, "Ride to this valley and bring me information about this man who claims to be a prophet, who says that revelation comes to him from the heavens. Listen to what he says, then come back to me."So the brother set out until he reached him and listened to his words, then returned to Abu Dharr and said, "I saw him commanding noble character, and speaking words that are not poetry."Abu Dharr replied, "You have not satisfied what I wanted to know."So he took provisions and carried a waterskin and travelled until he arrived in Mecca. He went to the sacred mosque and looked for the Prophet ﷺ, but he did not know him, and he was reluctant to ask about him, until part of the night had passed.Ali saw him and recognized that he was a stranger. When he saw him, he followed him, and neither of them asked the other about anything until morning came. Then Abu Dharr carried his waterskin and his provisions back to the mosque and spent that day there without the Prophet ﷺ seeing him, until evening came, when he returned to his sleeping place.Ali passed by him and said, "Isn't it time the man knew where to lodge?" So he made him get up and took him along with him, and still neither of them asked the other anything.When the third day came, Ali did the same again, and Abu Dharr stayed with him. Then Ali said, "Won't you tell me what brought you here?"Abu Dharr replied, "If you give me a covenant and pledge to guide me, I will."Ali did so, and Abu Dharr told him.Ali said, "It is truth, and he is the Messenger of Allah ﷺ. When morning comes, follow me. If I see anything I fear for you, I will stop as though I am pouring out water. If I continue, follow me until you enter where I enter."Abu Dharr did so. He went tracking his footsteps until Ali entered upon the Prophet ﷺ, and he entered with him. He listened to his words and accepted Islam on the spot.The Prophet ﷺ said to him, "Return to your people and inform them, until my command reaches you."Abu Dharr said, "By the One in whose hand my soul is, I will surely cry it out aloud among them."He went out until he came to the sacred mosque and called out at the top of his voice, "I bear witness that there is no god but Allah, and that Muhammad is the Messenger of Allah."Then the people rose up and beat him until they knocked him down. Abbas came and threw himself over him and said, "Woe to you. Don't you know that he is from Ghifar, and that the route of your traders to Syria passes through their territory?" So he rescued him from them.Then he returned the next day and did the same, and they beat him and rushed at him, but Abbas threw himself over him.

عن ابن عباس قال

لما بلغ أبا ذر مبعث النبي ﷺ قال لأخيه: اركب إلى هذا الوادي فاعلم لي علم هذا الرجل الذي يزعم أنه نبي، يأتيه الخبر من السماء، واسمع من قوله ثم ائتني، فانطلق الأخ حتى قدمه، وسمع من قوله، ثم رجع إلى أبي ذر، فقال له: رأيته يأمر بمكارم الأخلاق، وكلاما ما هو بالشعر. فقال: ما شفيتني مما أردت، فتزود وحمل شنة له فيها ماء حتى قدم مكة، فأتى المسجد، فالتمس النبي ﷺ، ولا يعرفه وكره أن يسأل عنه حتى أدركه بعض الليل، فرآه علي، فعرف أنه غريب، فلما رآه تبعه، فلم يسأل واحد منهما صاحبه عن شيء حتى أصبح، ثم احتمل قربته وزاده إلى المسجد، وظل ذلك اليوم ولا يراه النبي ﷺ حتى أمسى، فعاد إلى مضجعه، فمر به علي، فقال: أما نال للرجل أن يعلم منزله؟ فأقامه فذهب به معه، لا يسأل واحد منهما صاحبه عن شيء، حتى إذا كان يوم الثالث، فعاد علي مثل ذلك، فأقام معه، ثم قال: ألا تحدثني ما الذي أقدمك؟ قال: إن أعطيتني عهدا وميثاقا لترشدني فعلت، ففعل فأخبره، قال: فإنه حق وهو رسول الله ﷺ، فإذا أصبحت فاتبعني، فإني إن رأيت شيئا أخاف عليك قمت كأني أريق الماء، فإن مضيت فاتبعني حتى تدخل مدخلي ففعل، فانطلق يقفوه حتى دخل على النبي ﷺ ودخل معه، فسمع من قوله وأسلم مكانه. فقال له النبي ﷺ: ارجع إلى قومك، فأخبرهم حتى يأتيك أمري قال: والذي نفسي بيده، لأصرخن بها بين ظهرانيهم، فخرج حتى أتى المسجد، فنادى بأعلى صوته: أشهد أن لا إله إلا الله، وأن محمدا رسول الله. ثم قام القوم فضربوه حتى أضجعوه، وأتى العباس فأكب عليه، قال: ويلكم ألستم تعلمون أنه من غفار، وأن طريق تجاركم إلى الشام فأنقذه منهم، ثم عاد من الغد لمثلها، فضربوه وثاروا إليه، فأكب العباس عليه.