Hadith: From the gravest of lies is someone who ascribes himself to other than his biological father, or claims to have seen something in a dream which he actually never saw, or ascribes something to the Messenger of Allah (may Allah's peace and blessings be upon him) which he did not say

عن وَاثِلَةَ بن الأَسْقَعِ ـ رضي الله عنه ـ مرفوعاً: «إن من أعظم الفِرَى أن يَدَّعِيَ الرجلُ إلى غير أبيه، أو يُرِي عَيْنَهُ ما لم تَرَ، أو يقول على رسول الله صلى الله عليه وسلم ما لم يَقْلْ».
[صحيح] - [رواه البخاري]

Wāthilah ibn al-Asqa‘ (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "From the gravest of lies is someone who ascribes himself to other than his biological father, or claims to have seen something in a dream which he actually never saw, or ascribes something to the Messenger of Allah (may Allah's peace and blessings be upon him) which he did not say."
[Authentic hadith] - [Narrated by Bukhari]

Explanation

From the worst of lies in Allah's sight is that a man ascribes himself to someone who is not his biological father, or that someone else ascribes a man to other than his biological father and that man approves of such ascription. This is indeed a monstrous falsehood. Also from the worst types of lying in Allah's sight is that a person claims that he saw something in a dream which he never really saw. Another grave and severe type of lie in Allah's sight is to ascribe to the Prophet (may Allah's peace and blessings be upon him) something that he never said: did, or approved of.