Musnad Ahmad 397:2

Waki’ narrated to us, he said: Shu’bah narrated to us, from Yahya ibn Husayn, from his grandmother, who said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, saying: "May Allah have mercy on those who shave." They said upon the third time: "And those who shorten?" He said: "And those who shorten."