Musannaf Ibn Abi Shayba 3:3544

Abu Bakr narrated to us, he said: Muhammad ibn Fudayl narrated to us, from Layth, from Abd al-Rahman ibn al-Aswad, from his paternal uncle, who said: There was no hour in which I came to Abdullah ibn Mas’ud except that I found him praying between the Maghrib and the Isha [prayers]. And he used to say: "It is an hour of heedlessness."