Prostration is for God alone
Question:
If prostration is only for Allah, then why did God command the Angels to prostrate to Adam, and why in Surah Yusuf, we read that Yusuf's father and brothers prostrated to him?
Reply:
The act of prostration in worship should indeed be to God alone. However, the two cases you mentioned were not acts of worship, they were merely acts of prostration in response to God's command and God's will.
1- When the angels prostrated to Adam, they were not worshipping Adam, they were merely obeying a command from God. The source of the command (to prostrate to Adam) was God, and thus the angels, by obeying the command, were worshipping the source of the command (God).
2- Similarly, when the parents of Joseph and his brothers "fell prostrate before him" (12:100), they were not worshipping Joseph, but they did so in accordance with a preordained decree by God. That decree by God for that to happen was shown to Joseph in the vision he saw in his younger years (12:4).