Is prostration in prayer directly to Allah or to the Kaaba?

The prostration of a Muslim in prayer is for Allah the Almighty alone. The Kaaba is merely a direction that the worshiper has been commanded to face during all pillars of the prayer. Facing it is an act of compliance with Allah Almighty's command. If a Muslim is unable to determine the Qibla or face it, they may pray in any direction. It has been permitted for a traveler to perform supererogatory prayers on their mount, wherever it may be headed. Muslims initially prayed towards Jerusalem, then they were commanded to face the Kaaba. In both cases, the direction is wherever Allah Almighty directs. Allah Almighty said: "Say, 'To Allah belongs the East and the West. He guides whom He wills to a straight path'" and "And to Allah belong the East and the West. So wherever you [might] turn, there is the Face of Allah. Indeed, Allah is all-Encompassing and Knowing."