Many people refuse to acknowledge al-Azhar's fatwa on the permissibility of congratulating Christians on their occasions like Christmas and that this is a sunnah. They maintain that the Prophet (peace and blessings be upon him) did not do so with Jewish festivals and so we should not do so as well. They also asked for proofs from the Sunnah.

 

It is permissible to congratulate non-Muslims on their religious occasions with words that do not contradict the Islamic creed. Maintaining ties, giving gifts, visiting and congratulating non-Muslims are all instances of kind treatment. God Almighty commands us to speak kind words to everyone alike. He says, “Speak fair to people” (Quran 2: 83). He also says, “God commands justice and the doing of good” (Quran 16: 90).

 

The Prophet (peace and blessings be upon him) accepted gifts from non-Muslims
Authentic mass transmitted hadiths confirm that the Prophet (peace and blessings be upon him) accepted gifts from non-Muslims. He accepted the gift of al-Muqawqis, the great Christian ruler of Egypt. Ali Ibn Abu Talib (may God be pleased with him) reported that Chosroes, Caesar, and other kings sent gifts to the Prophet (peace and blessings be upon him) and he accepted them.Scholars of Islam understood from these hadiths the permissibility or recommendation of accepting gifts from non-Muslims because it is an act of benevolence. It is, moreover, a sunnah of the Prophet (peace and blessings be upon him).
The Prophet offered gifts to non-Muslims
Commenting on the gifts that the Prophet (peace and blessings be upon him) offered non-Muslims, Al-Sarkhasi said in Sharh Al-Siyar Al-Kabir (vol. 1, p. 96), "Offering gifts to others is from among good morals as per the words of the Prophet (peace and blessings be upon him), “I was sent to establish good manners.” Scholars therefore deduced from this hadith the recommendation of exchanging gifts between Muslims and non-Muslims.
In Fath Al-'Ali Al-Malik (vol. 2, p. 349), Sheikh 'Ilish was asked whether congratulating non-Muslims is considered apostasy. He replied, "Congratulating non-Muslims by wishing them a long life is not apostasy because it does not imply venerating or admitting disbelief.”
The ruling
The above noble verses, hadiths, and scholarly opinions, evince without doubt that maintaining ties with non-Muslims by exchanging visits and gifts, offering condolences and good wishes, and the like are all instances of kind treatment. This is considered one of the ways towards calling to the religion of God through noble manners.

And God Almighty knows best.