Hisham bin Amir reported,
I heard the Messenger of Allah ﷺ say, "It is not lawful for a Muslim to shun another Muslim for more than three nights. If they remain estranged beyond three, both of them have strayed from the truth as long as they continue in their estrangement. The first of them to return is the one whose return becomes his expiation. If he greets the other with peace and the other does not respond, the angels return the greeting to him while Satan responds to the other. And if they die while still estranged, they will never be united in Paradise."
Azami 14939
Sahihعن هشام بن عامر قال
سمعت رسول الله ﷺ يقول: لا يحل لمسلم أن يهجر مسلما فوق ثلاث ليال، فإن كان تصارما فوق ثلاث، فإنهما ناكبان عن الحق ما داما على صرامهما، وأولهما فيئا فسبقه بالفيء كفارته، فإن سلم عليه فلم يرد عليه ورد عليه سلامه ردت عليه الملائكة، ورد على الآخر الشيطان، فإن ماتا على صرامهما لم يجتمعا في الجنة أبدا.
Ahmad (16257), Bukhari in Adab al-Mufrad (402, 407), and Ibn Hibban (5664), who graded it sahih, all narrated it through chains from Yazid, from Muaza the Adawiyyah, who said, I heard Hisham bin Amir say, and then he mentioned it. Its chain is sahih. Haythami said in Majma (8/66), "The narrators of Ahmad are the narrators of the Sahih."Similar in meaning is what was narrated from Abu Hurairah, that the Prophet ﷺ said, "It is not lawful for a believer to shun a believer for more than three days. If three days pass, let him meet him and greet him with salaam. If the greeting is returned 
, then they both share in the reward, but if it is not returned to him, then he bears the sin."This was narrated by Abu Dawud (4912), Bukhari in Adab al-Mufrad (414), and Bayhaqi in Shuab (6195), all through chains from Muhammad bin Hilal bin Abu Hilal, who said, my father narrated to me, from Abu Hurairah, and then he mentioned it. Hilal bin Abu Hilal is majhul (unknown), and Ahmad said, "I do not know him." Despite this, Ibn Hajar said in Fath al-Bari (10/495), "Abu Dawud narrated it with a sahih chain." Had he, may Allah have mercy on him, said "a sahih hadith," it would have been understood in light of corroborating evidence.Similar in meaning is what was narrated from Abdullah bin Masud, who said, the Messenger of Allah ﷺ said, "If two men entered Islam and then shunned one another, the one who was in the wrong would be outside of Islam until he returns," meaning, the wrongdoer of the two.This was narrated by Bazzar (1773), Hakim (1/21–22), and Abu Naeem in Hilya (4/173), all through the route of Abdul-Samad bin Abdul-Warith, from Shuba, from Amash, from Zayd bin Wahb, from Abdullah bin Masud, who said, and then he mentioned it. Abu Naeem said, "It is gharib from the hadith of Amash, and Shuba only raised it as marfu from him through Abdul-Samad."Daraqutni was asked about this hadith, and he said in Ilal (721), "Amash and Talha bin Musarrif narrate it from Zayd bin Wahb. Abdul-Samad raised it as marfu from Shuba, from Amash, while others narrated it as mawquf, and the mawquf version is more likely correct."I say, Abdul-Samad bin Abdul-Warith al-Anbari is among the narrators of the Jamaah, Ibn Main graded him trustworthy, and Ibn Madini said, "He is firmly reliable in narrating from Shuba." So his being alone in narrating it does not harm it, except that his wording, "outside of Islam," contains a clear objectionable element.