As-salaamu'alaykum,
It was narrated from ash-Sha'bi that 'Ali (radiyAllahu anh) went out to them saying:
"How soothing it is to the heart."
It was said to him:
"What is that?"
He said:
"To say, when you do not know something: Allah (subhaanahu wa ta'aala) knows best."
(Jami' bayna al-'Ilm wa Fadlihi, 2/66)
It was narrated from ash-Sha'bi that 'Ali (radiyAllahu anh) went out to them saying:
"How soothing it is to the heart."
It was said to him:
"What is that?"
He said:
"To say, when you do not know something: Allah (subhaanahu wa ta'aala) knows best."
(Jami' bayna al-'Ilm wa Fadlihi, 2/66)