Praise be to Allah.
With regard to the first isnaad, its men are thiqaat (trustworthy), the men of al-Bukhaari and Muslim, but Mugheerah – whose full name is Mugheerah ibn Miqsam ad-Dabbi – is mudallis [i.e., he engaged in tadlees, which is when a narrator narrates a hadith that he did not hear directly from his shaykh, without mentioning the name of the third party from whom he did hear it, using wording that may or may not give the impression that he heard it directly] and he narrated it by saying ‘an [meaning “from”, instead of saying “So and so told us” or “I heard So and so saying” etc] from Abu Waa’il and Ibraaheem.
See: Tadhdheeb at-Tahdheeb (10/241); ath-Thiqaat by Ibn Hibbaan (7/464).
What appears to be the case is that it is acceptable here because reports narrated by Mugheerah from [‘an] Abu Wa’il and Ibraaheem were narrated in as-Saheehayn, in marfoo‘ reports, and the reports here is mawqoof, and undoubtedly leniency is more acceptable in mawqoof reports.
With regard to the second isnaad, its men are also the men of al-Bukhaari and Muslim. With regard to Abu Mu‘aawiyah, although more than one of the scholars had some reservations about him, their reservations were only with regard to reports other than the reports he narrated from al-A‘mash. As for the reports he narrated from al-A‘mash in particular, he used to have a good memory of them, and he is one of the most trustworthy companions of al-A‘mash. Wakee‘ said: We have not met anyone who was more knowledgeable of the reports narrated by al-A‘mash than Abu Mu‘aawiyah.
See: at-Tahdheeb (9/121).
Mugheerah differed from al-A‘mash. Mugheerah narrated it from Abu Waa’il – with corroborating reports from Ibraaheem, who is an-Nakha‘i – from ‘Umar, and al-A‘mash narrated it from Abu Waa’il – whose name is Shaqeeq ibn Salamah – from ‘Amr ibn Shurahbeel from ‘Umar. The hadith of al-A‘mash is more sound.
Whatever the case, it is narrated soundly from ‘Umar, whether via Mugheerah or via al-A‘mash.
With regard to the report of al-Hasan ibn ‘Ali, its isnaad includes Jahsh whose full name is Jahsh ibn Ziyaad ad-Dabbi. Ibn Abi Haatim mentioned him in al-Jarh wa’t-Ta‘deel (2/550), but he did not mention any details regarding him. Al-Bukhaari also mentioned him in at-Taareekh (2/253) and said: ath-Thawri, Abu Bakr and Muhammad ibn Fudayl ibn Ghazwaan narrated from him. End quote.
Ibn Hibbaan mentioned him in ath-Thiqaat (6/157).
Therefore not much is known about him. Some scholars regard as hasan the hadith of narrators such as this, especially mawqoof reports such as this.
The ruling on rennet varies according to its source. If it is taken from an animal that was slaughtered in the prescribed manner, then it is pure (taahir) and may be eaten. If it is taken from an animal that died of natural causes or from an animal that was not slaughtered in the prescribed manner, then there is a difference of opinion among the fuqaha’ regarding that. The majority of the Maalikis, Shaafa‘is and Hanbalis are of the view that it is najis (impure). Abu Haneefah and Ahmad, according to another report from him, are of the view that it is taahir (pure). This is the view favoured by Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him), who said in al-Fataawa (21:103):
What is more likely to be the case is that their – that is, the Magians’ – cheese is halaal and that the rennet and milk of an animal that died of natural causes are taahir. That is because when the Sahaabah conquered Iraq, they ate the cheese of the Magians, and this was the prevalent practice among them. What is narrated from some of them, that they regarded that as makrooh, is subject to further examination.
See questions no. 2841 and 115306.
And Allah knows best.