Musnad Ahmad 30:773

Ayyub ibn al-Najjar narrated to us, from Tayyib ibn Muhammad, from ‘Ata’ ibn Abi Rabah, from Abu Hurayrah, who said: "The Messenger of Allah (may Allah bless him and grant him peace) cursed the effeminate men who imitate women, and the masculine women who imitate men, and the celibate men who say: 'We will not marry,' and the celibate women who say that, and the one who travels through the wilderness alone." This weighed heavily upon the companions of the Messenger of Allah (may Allah bless him and grant him peace), until it became apparent on their faces. He then said: "The one who spends the night alone."