What is the authenticity of the hadith "if you eat meat for 40 days, those qualities will come within you"?

Question

In my answer to How often should we eat meat?, I quote two ahadith which I'm unsure about their reliability. The second is

“At one time the Rasul of Allah said to his cousin ‘Ali, ‘O ‘Ali, you should not eat meat. If you eat meat for 40 days, those qualities will come within you. Because of that, your human qualities will change, your compassionate qualities will change, and the essence of your body will change.’ “
_{Islamic Quotes}

Another version is:

The Prophet (s.a.w.s.) reportedly said that one who does not eat meat for forty consecutive days will waste away, whereas to eat meat for forty consecutive days will harden the heart. In other words, one should moderate the intake of meat.
_chishti.org

This one actually mentions two ahadith: one about eating meat for 40 days, and one about not eating meat for 40 days.

I didn't find a reliable source for this hadith, nor was I able to determine its level of authenticity.

Question: What is the authenticity of this hadith?

(I asked about the other hadith here: What is the authenticity of the hadith "do not make your stomach a graveyard of animals"?)

Answer 1

The exact words that you quoted are not in any Sunni book that I have come across. However, a hadith attributed to 'Ali ibn Abi Tālib — not to the Prophet — was documented by Muhammad al-Munawi in Fayd al-Qadīr:

قال علي كرم الله وجهه: من ترك اللحم أربعين يوما ساء خلقه ومن داوم عليه أربعين يوما قسا قلبه

_{— NOTE: My own translation, so treat with care.}

'Ali, may Allah honor his face, said: "Whoever forsakes eating meat for 40 days, his manners will become bad; whereas, whoever eats meat for 40 days, his heart will harden.

_{— Fayd al-Qadīr Sharh al-Jami' as-Saghīr 4/124}

The hadith or variations thereof were mentioned in several books, e.g., in Ihyā' 'Ulūm ad-Dīn (3/95) by Abu Hamid al-Ghazali attributed to 'Ali, and in Islāh al-Māl (2/69) unattributed in tamrīd form (i.e., in passive voice, which in this case was "it was told that ...") by Ibn Abi ad-Dunya. Worth noting that Fayd al-Qadīr is actually quoting Ihyā' 'Ulūm ad-Dīn.

The hadith, as Al-Munawi said, was considered mawdū' (fabricated) by Ibn al-Jawzi in his book A Great Collection of Fabricated Traditions. This is due to having Sulaiman An-Nakh'i (Arabic: سليمان النخعي) in its chain of narration, and he was known to be a liar as documented in Az-Ziyādāt 'ala al-Mawdū'āt (3/555) by As-Suyūti.