When is it permissible for us to say, "Allah and His Messenger know best"?
Machine translationRead the Arabic original
The questioner : During the life of the Messenger of Allah (peace and blessings be upon him) and after the death of the one to whom the phrase "Allah and His Messenger" refers (peace be upon him).
The Shaykh : Yes, in matters of the Shari'ah, in matters of the Shari'ah.
The questioner : May Allah reward you.
The Shaykh : After his death, yes, only in matters of the Shari'ah. In worldly matters, if, for example, someone asks, "Will so-and-so attend tomorrow?" you do not say, "Allah and His Messenger know best," because the Messenger has no knowledge of that. But in matters of the Shari'ah, Muhammad (peace and blessings be upon him) is the most knowledgeable of creation regarding them.
The Shaykh : Yes, in matters of the Shari'ah, in matters of the Shari'ah.
The questioner : May Allah reward you.
The Shaykh : After his death, yes, only in matters of the Shari'ah. In worldly matters, if, for example, someone asks, "Will so-and-so attend tomorrow?" you do not say, "Allah and His Messenger know best," because the Messenger has no knowledge of that. But in matters of the Shari'ah, Muhammad (peace and blessings be upon him) is the most knowledgeable of creation regarding them.
Source
قناة «فضيلة الشيخ محمد بن صالح العثيمين»، 3260 - 4600 متى يجوز أن نقول الله ورسوله أعلم؟ ابن عثيمين — youtube.com
The Arabic text is a machine transcription of the audio recording.