Silsilat al-Ahadith al-Sahihah

Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates that the Messenger of Allah (peace and blessings be upon him) was mentioned in the presence of the Magians. The Prophet (peace and blessings be upon him) said: "These (Magians) let their mustaches grow and shave their beards, so you should oppose them." Thus, Sayyiduna Ibn Umar (may Allah be pleased with them both) used to trim his mustache in such a way as the hair of a goat or camel is trimmed.