Our Prophet (saas) revealed that for someone to say he had a dream that he did not is a lie

Ibn Umar narrates:

The worst lie is for someone to say he had a dream that he did not.

Bukhari, Tabir: 45, Musnad 2:96, 119
Reference: Jami’us Sagir / (A translation based on the Feyzul Kadir collected by Suyuti)

In the Qur’an, Allah makes it unlawful to lie. Our prophet (saas) reveals in this hadith that it is a lie to for someone to say he had a dream that he did not, in the same way that it is a lie for someone to say he did not do something that he did, or to deny saying something that he did say. Indeed, our Prophet (saas) stated this was the very worst lie. Telling people that describing a dream one has not had as if one had experienced it is unlawful, and warned them against such a sin.

In one verse of the Qur’an Allah says that lying will prevent a person attaining salvation:
 

Allah does not guide any unbridled inveterate liar." (Surah Ghafir, 28)

In the same way there is a rise in all sins in the End Times in which we are living, it has also become common for people to tell another they dreamed things that they did not. So much so that in another hadith our Prophet (saas) says that unlawful things will start being regarded as lawful in the End Times:

    "Unlawful things being regarded as lawful ... is one of the signs of the Last Day." (Ölüm-Kıyamet-Ahiret ve Ahirzaman Alametleri, p. 454