The Hadith: 'Much laughter kills the heart'
Are the following two statements Hadiths or Athar (narrations from the Companions): “May Allah be merciful to the one who knows his own limits” and “Much laughter kills the heart” Who are their narrators and to what extent are they Sahih (authentic)?
Research and verification show that the phrase that you have mentioned in the question reading: “May Allah be merciful to the one who knows his own limits” has no origin in any of the variant narrations. As far as we know, such a phrase is not found in the authentic Books of Hadith scholars and it is not reported from any of the Sahabah (Companions of the Prophet) or the Tabi`un (Followers, the generation after the Companions of the Prophet). It is a commonly used phrase whose sayer is unknown.
On the other hand, the Hadith which is quoted as reading: “Much laughter kills the heart” is related by Ibn Majah in his Sunan (Hadith compilations classified by jurisprudential themes) on the authority of Abu Hurayrah (may Allah be pleased with him) as reading: The Messenger of Allah (peace be upon him) said: “Do not laugh a lot for much laughter kills the heart.
Al-Busiry said in his book Al-Zawa’id: “The Sanad (chain of narrators) of this Hadith is Sahih and its narrators are Thiqah (trustworthy).”
Moreover, Al-Quda’i and Al-Tabarani related a Hadith similar to it which is related as well by Ibn Majah on the authority of Wathilah ibn Al-Asqa’ from Abu Hurayrah who said that the Messenger of Allah (peace be upon him) said: “O Abu Hurayrah, be pious and you will be the most devoted worshipper, be content and you will be the most thankful person, love for people what you love for yourself and you will be a true believer, do good to your neighbors and you will be a perfect Muslim, and laugh only a little for much laughter kills the heart.”
Al-Busiri commented in his book Al-Zawa’id: “This Hadith has a good Sanad.”
Imam Ahmad, Al-Tirmidhi, Al-Bayhaqi, and others related a Hadith similar to the foregoing but Al-Tirmidhy said: “It is a Hadith Gharib (a Hadith with a single narrator usually at the beginning of the chain of narration).”
The Hadith concerned has many other Sanads (chains of narrators). Consequently, it is at least regarded as Hadith Hasan (a Hadith whose chain of narration contains a narrator with weak exactitude, but is free from eccentricity or blemish).
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.