The distance between the ears of the Dajjal's donkey is forty cubits

It is narrated from Sayyiduna Jabir (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "The Dajjal will emerge when there is weakness in religion and knowledge has turned its back. He will have forty days to roam the earth; one day of them will be like a year, one day like a month, one day like a week, and the rest of his days will be like your ordinary days. He will have a donkey which he will ride; the width between its two ears will be forty cubits. He will come to the people and say: I am your Lord! But your Lord is not one-eyed. Between his two eyes will be written 'K F R', which every believer will read, whether he knows how to write or not. He will pass by every water and watering place except Madinah and Makkah; Allah Almighty has made these two cities forbidden to him, and the angels will be guarding their gates."
This hadith is sahih (authentic) on the condition of Imam Muslim, but the two Shaykhs (Bukhari and Muslim) did not record it.

Jubayr bin Nufayr narrates from his father that the Messenger of Allah (peace and blessings be upon him) mentioned the Dajjal and said: "If he (the Dajjal) emerges while I am present, then I will be the one who contends with him on your behalf; and if he emerges after me, then every person will be his own defender, and Allah is my successor (guardian) over every Muslim. Beware! He is one-eyed, as if his eye were like the eye of Abd al-Uzza bin Qatan al-Khuza'i. Take heed! Between his two eyes will be written 'Kafir' (disbeliever), which every Muslim will read. So whoever of you meets him should recite the opening verses of Surah al-Kahf. He will emerge from the route between Syria and Iraq and will spread great mischief to the right and to the left. O servants of Allah, remain steadfast!" He (peace and blessings be upon him) said this three times. It was said: O Messenger of Allah! How long will he stay on the earth? He (peace and blessings be upon him) said: "Forty days: one day like a year, one day like a month, one day like a week, and the rest of the days will be like your ordinary days." The Companions said: O Messenger of Allah! How shall we pray on that (long) day? Will the prayers of a single day suffice, or shall we estimate the time? He (peace and blessings be upon him) said: "No, rather you should estimate (the time)."
This hadith has a sahih (authentic) chain of narration, but the two Shaykhs (Bukhari and Muslim) did not record it.