Musnad Ahmad 1031:2

Waki’ narrated to us, Shu’bah narrated to us, from Yahya ibn Husayn, from his grandmother, who said: I heard the Messenger of Allah, may Allah bless him and grant him peace, saying: "May Allah have mercy on those who shave [their heads], may Allah have mercy on those who shave, may Allah have mercy on those who shave." They said on the third time: "And those who shorten [their hair]?" He said: "And those who shorten."