Is it true that Prophet Muhammed, sallallaahu 'alayhi wa sallam, said that we should honour an Islamic scholar whenever we meet one at an occasion? Which hadith says that?

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad is His slave and Messenger.

We have not come across a hadeeth with the wording that you mentioned in the question, but there is general evidence which states that we have to respect and honor the scholars who know about Allaah and about His Commandments, and this is something encouraged in the sharee'ah.

Al-Haakim said in Al-Mustadrak: Chapter: Respecting the scholar, “These are authentic texts about the command to respect the scholar when attending his gathering and sitting before him.” Then he mentioned some ahaadeeth, among which are the following:

Ubaadah ibn As-Saamit narrated that the Prophet said, “He is not one of us, who does not honor our elderly, who does not have mercy towards our young, and who does not value our scholars.” [Ahmad in his Musnad and others - Al-Albaani classified it as hasan (good)]

Al-Bayhaqi said in Al-Madkhal ila As-Sunan: ‘Chapter on honoring the scholars and knowledge’, and he mentioned the aforementioned hadeeth and others, including:

Abu Moosa Al-Ash'ari narrated that the Prophet said, “Part of glorifying Allaah is honoring the grey-haired (i.e. the elderly) Muslim, the one who has learned the Quran by heart without exaggerating about it or ignoring its teachings, and the just ruler.” [Abu Daawood - al-Albaani classified it as hasan]

Abu Umaamah narrated that the Prophet said, “There are three things that are part of glorifying Allaah: honoring the grey-haired (i.e. the elderly) Muslim; the one who has learned the Quran by heart; and the one who has knowledge be he young or old.” Al-Bayhaqi classified it as weak by saying: Khalid ibn Yazeed is Ibn ‘Abdur-Rahmaan ibn Maalik Ash-Shaami, and he is not among the strong narrators.

Besides, Ibn Taawoos narrated from his father who said, “It is part of the Sunnah to honor four persons: a scholar, an elder, a leader (ruler), and a father. It is considered rude that a man call his father by his name.”

Allaah knows best.