Musnad Ahmad 1243:3

Waki’ narrated to us, he said: Shu’bah narrated to us, from Yahya ibn al-Husayn, from his grandmother, who said: I heard the Prophet, peace and blessings of Allah be upon him, while he was saying: "May Allah have mercy on those who shave [their heads], may Allah have mercy on those who shave [their heads]." They said at the third time: "And those who shorten [their hair]?" He said: "And those who shorten [their hair]."