Abu Sa'eed Al-Khudri narrated that:
The Messenger of Allah said: "People will come to you seeking knowledge. When you see them say to them, 'Welcome, welcome,' in obedience to the injunctions of the Messenger of Allah and instruct them in knowledge."(One of the narrators said) "I said to Al-Hakam: 'What is 'Iqnuhum?' He said: 'Instruct them.'"
English reference: Vol. 1, Book 1, Hadith 247
Arabic reference: Book 1, Hadith 255