Abdur-Rahman Al-Awzaa’ee 157 AH (May Allah have mercy on him) was asked if Imaan increases. He said:

نعم حتى يكون كالجبل

“Yes, until it becomes like a mountain”

He was then asked if it decreases and he said:

نعم حتى لا يبقى منه شيء

“Yes, until there remains nothing from it”

Taken from: Asbaab Ziyaadatil Imaan wa Nuqsaanihi p. 6