Charles Ellicott Commentary Romans 8:27

"and he that searcheth the hearts knoweth what is the mind of the Spirit, because he maketh intercession for the saints according to [the will of] God." — Romans 8:27 (ASV)

God recognizes the voice of His own Spirit, because the prayers that the Spirit prompts are in strict accordance with His will.

What is the mind of the Spirit.—What are the thoughts of the Spirit, and therefore what is the echo of those thoughts in the prayers that are offered to Him.