‘Abdallah [b. Mas'ud] said:
If any of you people knows anything he should
make it known, but if anyone does not know he should say that God
knows best, for saying this when one does not know is a part of knowledge. God said to His prophet, “Say, I do not ask you for any reward
for it, and I am not a pretender.”* (Bukhari and Muslim.)
*Qur’an, xxxviii, 86.