Assalamualaikum ustaz. What is meant by “إِنَّ مِنَ البَيَانِ لَسِحْرًا”?
Waalaikumussalam wrt. wbt.,
Alhamdulillah, praise and gratitude to Allah SWT for His countless blessings for us all. Praise and salutations to our beloved Prophet Muhammad PBUH, his family, companions, and all those who follow his footsteps until the Final day.
There is a hadith narrated by Ibn Umar R.Anhuma, he said: “Two men from the east stood up and spoke, and people were amazed at their eloquence. The Messenger of Allah, may Allah bless him and grant him peace, said:
إِنَّ مِنَ البَيَانِ لَسِحْرًا
“Part of eloquence is sorcery.” 
Sorcery in this context doesn’t mean prohibited sorcery, rather it means the beauty in words that enchants the listeners and captivates them.
Ibn Kathir in his commentary  stated when interpreting sorcery in the above verse of surah al-Baqarah, presented this hadith and said: This hadith may be meant as praise as stated by an opinion of a group. It may mean a rebuke towards balaghah and this according to al-Qurtubi is the more accurate opinion, for balaghah allows the batil.
This is as narrated from Aisyah R.Anha, Rasullullah PBUH said:
فَلَعَلَّ بَعْضًا أنْ يَكونَ أبْلَغَ مِن بَعْضٍ، أقْضِي له بذلكَ وأَحْسِبُ أنَّه صادِقٌ
“And maybe one of them presents his case eloquently in a more convincing and impressive way than the other, and I give my verdict in his favor thinking he is truthful.” 
Ibn Battal in his syarah of Sahih al-Bukhari stated: “it means that it functions to enchant the souls of people just as sorcery tempts them. Sorcery here is meant as a comparison, the wrong and prohibited isn’t meant in this context. Wallahu a’lam.” 
Ibn al-Athir said: Indeed, from the speech is sorcery, turning the hearts of people listening, even if it is untrue. It is also said that it means the speech leads to sin just as the sin committed by the sorcerer with his magic, it may mean an insult, and it may also mean praise. The reason is speech al-bayan softens the hearts, cools anger and removes hardships. Enchanting speech means, changing something from reality.” 
Whereas Syeikh Muhammad bin Soleh said: Al-Bayan is sorcery (enchanting) for it mellows the hearts of listeners and turns them or makes the listeners sympathize with it. The listeners assume that falsehood as the truth, and for the great influence of the preacher, the listeners are charmed by his speech. This is the reason if a person speaks with false words, however, due to his eloquence, he enchants his listeners and they are charmed and mesmerized by him. This is a type of sorcery named eloquence and charm. Al-Bayan is a result of mellowness and charm. Al-Bayan is actually another word for eloquence and there is no doubt that it is a sort of sorcery Ibn al-Qayyim said al-Hur (الحور): His speech is permissible sorcery. 
Lastly, we state that al-bayan or enchanting speech is a type of sorcery, however, it is not black magic that is prohibited by syarak but it means the beauty of speech that is able to mesmerize its listeners and charms them so much so that he can uphold the truth or falsehood. However, although if a person is blessed by the eloquence of speech and is able to enchant his listeners, however, Islam prohibits him from using it to spread false beliefs using his speech that he turns people away from the truth.
We also remind anyone with this gift of eloquence in speech to use it to uphold the truth about Islam and fight against falsehood following the guidelines of syarak. May Allah SWT grant us all understanding in this religion. Amin.
 Narrated by al-Bukhari (5767)
 See Tafsir Ibn Kathir, 1/255
 Narrated by al-Bukhari (7185)
 See Syarh Sahih al-Bukhari Li Ibn Battal, 9/448
 See al-Nihayah fi Gharib al-Hadith wa al-Athar, 2/346
 See Syarh Kitab al-Tauhid, 1/ 527