“The best supplication is the supplication on the Day of ʿArafah, and the best thing that I and the prophets before me have said is: la ilaha illa Allah (there is no god but Allah).”
I say: This is a mursal report with a good chain of transmission. Al-Muttalib is Ibn ʿAbd Allah ibn Hantab, who was truthful, and those below him are trustworthy narrators used by Muslim, apart from Abu Marwan—Muhammad ibn ʿUthman ibn Khalid al-Umawi—who was truthful but made mistakes, as al-Hafiz stated in al-Taqrib.
In sum, the hadith is established through the combined strength of these supporting reports. And Allah knows best.
1504 — “The best martyrs are those whose blood is shed and whose steeds are hamstrung.”
Ahmad narrated it (5/265) through ʿAli ibn Yazid, from al-Qasim Abu ʿAbd al-Rahman, from Abu Umamah, who said: “... I said: ‘O Prophet of Allah, which martyrs are best?’ He said: ‘Those whose blood is shed ...’”
I say: This is a weak chain of transmission. ʿAli ibn Yazid—who is al-Alhani—was declared “weak” by al-Hafiz.
It has a supporting report narrated by Ibrahim ibn Hisham ibn Yahya ibn Yahya al-Ghassani: my father narrated to us, from my grandfather, from Abu Idris al-Khawlani, from Abu Dharr, attributed to the Prophet, with similar wording. Ibn Hibban narrated it (94). However, this Ibrahim was a liar, so it is not suitable as corroborative evidence.
Nevertheless, the hadith is authentic, for it has many supporting reports. Among them is the report from ʿAbd Allah ibn Hubshi al-Khathʿami that the Prophet, peace and blessings be upon him, was asked, “Which manner of being killed is most noble?” He replied: “That in which one’s blood is shed and one’s steed is hamstrung.”