Muhammad pbuh can command only what Allah commands to Him

Why?

1. Because Command belongs to none but Allah

Say: "For me, I (work) on a clear sign from my Lord, but ye reject Him. What ye would see hastened, is not in my power. The command rests with none but Allah. He declares the truth, and He is the best of judges." 6/57

"If not Him, ye worship nothing but names which ye have named,- ye and your fathers,- for which Allah hath sent down no authority: the command is for none but Allah. He hath commanded that ye worship none but Him: that is the right religion, but most men understand not... 12/40

Further he said: "O my sons! enter not all by one gate: enter ye by different gates. Not that I can profit you aught against Allah (with my advice): None can command except Allah. On Him do I put my trust: and let all that trust put their trust on Him." 12/67

2. Allah does not share His command with anyone

Say: "(Allah) knows best how long they stayed: with Him is (the knowledge of) the secrets of the heavens and the earth: how clearly He sees, how finely He hears (everything)! They have no protector other than Him; nor does He share His Command with any person whatsoever. 18/26

Following commands of anyone that are not Allah’s commands or according to Allah’s commands, is SHIRK.