Al-Mundhiri recorded the following chapter in his book Al-Targhib wal-Tarhib:

ترهيب ذي الوجهين وذي اللسانين

Discouragement to carry two-faces and to speak with a forked-tongue.

Source: al-Targhīb wal-Tarhīb 3/602

Abu Huraira reported: The Messenger of Allah, peace and blessings be upon him, said:

‏‏إِنَّ مِنْ شَرِّ النَّاسِ ذَا الْوَجْهَيْنِ الَّذِي يَأْتِي هَؤُلاَءِ بِوَجْهٍ وَهَؤُلاَءِ بِوَجْهٍ

Verily, among the worst of people is one with two faces, he who comes with one face to these and another to those.

Source: Ṣaḥīḥ al-Bukhārī 6757, Grade: Muttafaqun Alayhi

Muhammad ibn Zayd reported: Some people said to Ibn Umar, may Allah be pleased with him, “When we had an audience with a ruler, we would speak to them differently than what we said after we left them.” Ibn Umar said:

كُنَّا نَعُدُّهَا نِفَاقًا

We used to consider that hypocrisy.

Source: Ṣaḥīḥ al-Bukhārī 6756, Grade: Sahih

‘Ammar reported: The Messenger of Allah, peace and blessings be upon him, said:

مَنْ كَانَ لَهُ وَجْهَانِ فِي الدُّنْيَا كَانَ لَهُ يَوْمَ الْقِيَامَةِ لِسَانَانِ مِنْ نَارٍ

Whoever carries two faces in the world, he will have two tongues of fire on the Day of Resurrection.

Source: Sunan Abī Dāwūd 4873, Grade: Sahih

Anas reported: The Messenger of Allah, peace and blessings be upon him, said:

مَنْ كَانَ لَهُ لِسَانَانِ فِي الدُّنْيَا جَعَلَ اللَّهُ لَهُ لِسَانَيْنِ فِي نَارِ يَوْمِ الْقِيَامَةِ

Whoever speaks with a forked-tongue in the world, Allah will make two tongues of fire for him on the Day of Resurrection.

Source: Musnad Abī Ya’lá 2746, Grade: Sahih li ghayrihi