Who said: "I never saw a day more beautiful or brighter than the day the Messenger was born, nor a day more abominable or darker than the day the Messenger died"?

The speaker is Anas ibn Malik (may Allah be pleased with him). Ibn al-Qayyim, in "Zad al-Ma'ad," quoted him as saying: "I witnessed him the day he entered Medina, and I never saw a day more beautiful or brighter than the day he entered Medina among us. And I witnessed him the day he died, and I never saw a day more hideous or darker than the day he died (peace and blessings of Allah be upon him)." This hadith was narrated by Al-Darimi in his Sunan, Imam Ahmad, and others.

Imam Ahmad and Al-Tirmidhi narrated – and Al-Bayhaqi graded it good – with a chain of transmission that Ibn Kathir described as being "on the conditions of the two Shaykhs [Bukhari and Muslim]," from Anas (may Allah be pleased with him), who said: "When it was the day the Messenger of Allah (peace and blessings of Allah be upon him) entered Medina, everything in it became illuminated. And when it was the day he died, everything in Medina became dark." In one narration: "Medina became so dark that some of us could not see others, and one of us would stretch out his hand and not be able to see it." In another narration: "So I never saw a day uglier than it, and we had not yet finished burying him when we found our hearts disbelieving."