Musnad Abu Ya'la al-Mawsili 165:4

Ya'qub ibn 'Isa, the neighbor of Ahmad ibn Hanbal, narrated to us: Ibrahim ibn Sa'd narrated to us, from 'Abd al-'Aziz ibn al-Muttalib, from 'Abd al-Rahman ibn al-Harith ibn 'Abd Allah, from Zayd ibn 'Ali ibn Husayn, from his father, from his grandfather, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Whoever is killed defending his right, he is a martyr."