The narration by Al nu'man ibn Bashir about the least form of punishment in jahannam is a muttafaqun alayhi Hadith. Meaning it is mentioned in sahih bukhari and Muslim, thus it reaches a very high level of authenticity.[another similar narration can be found in tirmidhi as well (book 39 Hadith 36)]
The narration by Samurah b. Jundub can be found in sahih Muslim. Infact there are two narrations with a slight difference in wording. [ Muslim 2845a & 2845b ( book 53 Hadith 38 & 39 ) ]
(You can find the translated text as well as Arabic on www.sunnah.com)
REGARDING THE LEAST FORM OF PUNISHMENT
i have not come across any Hadith that explicitly states the least form of punishment in jahannam other than the narration by ibn Bashir. Though there are many reports about the various degrees and severity of punishments, this Hadith is the one that states the least form/punishment.
The narration from Samurah tends to give us a notion of such but scholars have given more clarity on it. I will quote what I found relevant regarding this, inshallah.
- regarding the narration of Samurah :
Al-Haafiz (may Allah have mercy on him) said : It may be that this was narrated concerning those among the people of Tawheed who enter the Fire, and the circumstances of their punishment will vary according to their deeds. As for the disbelievers, they will be fully immersed in the fire.[ Fath al-Baari (11/394) ]
Al-Qurtubi said, “This topic indicates that the kufr of the person who simply disbelieves is not like the kufr of a person who oppresses, disbelieves, rebels and disobeys. There is no doubt that the infidels will suffer different levels of punishment in hell, as is known from Qur’an and Sunnah. We know for certain that the punishment of those who oppressed and killed the Prophets and Muslims, and spread corruption and disbelief on earth, will not be the same as the punishment of those who simply disbelieved, but treated the Prophets and Muslims well. Do you not see Abu Talib, how the Prophet (peace be upon him) will bring him up to a shallow level of Hell, because he helped him, protected him and treated him well? The Hadith narrated by Muslim from Samurah may be interpreted as referring to the infidel, as there is a Hadith about Abu Talib, or it may be interpreted as referring to those monotheists who will be punished.” (At-Tadhkirah, p. 409)
With this we are able to have an understanding as which is the least form of punishment in jahannam. As punishments are dealt accordingly, there are all the more possibility that it can be different for the people of tawhid and others. May Allah protect us all.
NOTE: there are two more narrations regarding less severe form of punishments reported by ibn Haatim in his tafseer and another narrated by Al-Hakeem at Tirmidhi in Nawaadir ul-usool but i find it's not relevant to mention those in detail because those are not authentic.
May Allah the mighty and sublime grant us mercy and guide us to the straight path.
Allah knows best