A.T. Robertson Commentary Mark 14:30

"And Jesus saith unto him, Verily I say unto thee, that thou to-day, [even] this night, before the cock crow twice, shalt deny me thrice." — Mark 14:30 (ASV)

Twice (δις). This detail only in Mark. One crowing is always the signal for more. The Fayum papyrus agrees with Mark in having δις. The cock-crowing marks the third watch of the night .