Musnad Ahmad 1153:2275

Ubayd ibn Abi Qurrah narrated to us, he said: Sulayman—meaning Ibn Bilal—narrated to us, from Alqamah, from his mother, in a story she mentioned, and she said: Aisha said: I heard the Messenger of Allah, may Allah bless him and grant him peace, say: "Whoever points a piece of iron at any of the Muslims, intending to kill him, then his blood has become obligatory."