It is narrated from the first Shia Imam, Ali ibn Abitaleb (AS) who said: No servant of God will taste the flavour of belief unless he/she abandons telling lies whether humorously (joking) or seriously [1]. In this regard, Imam Khomeini (RA) said: One of the cases that the human being must be careful is that telling a lie even as a joke has been prohibited in narrations and has been seriously commanded to abandon it, and the religious scholars have also issued Fatwas, declaring it (telling a lie) as Haram [2].
[1] Osul Kafi, Vol. 4, Hadith 11.
[2] Chehel Hadith, Imam Khomeini (RA), Page 472.